By JIM IRWIN, Associated Press Writer Jim Irwin, Associated Press Writer - 13 mins ago

ROMULUS, Mich. - An attempted terrorist attack on a Christmas Day flight began with a pop and a puff of smoke - sending passengers scrambling to subdue a Nigerian man who claimed to be acting on orders from al-Qaida to blow up the airliner, officials and travelers said.

The commotion began as Northwest Airlines Flight 253, carrying 278 passengers and 11 crew members from Amsterdam, prepared to land in Detroit just before noon Friday. Travelers said they smelled smoke, saw a glow, and heard what sounded like firecrackers. At least one person climbed over others and jumped on the man, who officials say was trying to ignite an explosive device.

"It sounded like a firecracker in a pillowcase," said Peter Smith, a passenger from the Netherlands. "First there was a pop, and then (there) was smoke."

Smith said one passenger, sitting opposite the man, climbed over passengers, went across the aisle and tried to restrain the man. The heroic passenger appeared to have been burned.

Afterward, the suspect was taken to a front-row seat with his pants cut off and his legs burned. Multiple law enforcement officials also said the man appeared badly burned on his legs, indicating the explosive was strapped there. The components were apparently mixed in-flight and included a powdery substance, multiple law enforcement and counterterrorism officials said.

The White House said it believed it was an attempted act of terrorism and stricter security measures were quickly imposed on airline travel. Dutch anti-terrorism authorities said the U.S. has asked all airlines to take extra precautions on flights worldwide that are bound for the United States.

The incident was reminiscent of Richard Reid, who tried to destroy a trans-Atlantic flight in 2001 with explosives hidden in his shoes, but was subdued by other passengers.

Multiple law enforcement officials identified the suspect in Friday's attempted attack as Umar Farouk Abdul Mutallab. He was described as Nigerian.

One law enforcement official said the man claimed to have been instructed by al-Qaida to detonate the plane over U.S. soil, but other law enforcement officials cautioned that such claims could not be verified immediately, and said the man may have been acting independently - inspired but not specifically trained or ordered by terror groups.

All the officials spoke on condition of anonymity because the investigation was continuing.

Intelligence and anti-terrorism officials in Yemen said they were investigating claims by the suspect that he picked up the explosive device and instructions on how to use it in that country. The officials spoke on condition of anonymity because they are not authorized to speak to the media.

The man was being questioned Friday evening. An intelligence official said he was being held and treated in an Ann Arbor, Mich., hospital. The hospital said one passenger from the flight was taken to the University of Michigan Medical Center in Ann Arbor, but referred all inquiries to the FBI.

Melinda Dennis, who was seated in the front row of the plane, said the man involved was brought to the front row and seated near her. She said his legs appeared to be badly burned and his pants were cut off. She said he was taken off the plane handcuffed to a stretcher.

One law enforcement official, also speaking on condition of anonymity, said Mutallab's name had surfaced earlier on at least one U.S. intelligence database, but he was not on a watch list or a no-fly list.

The suspect boarded in Nigeria and went through Amsterdam en route to Detroit, Rep. Peter King, the ranking GOP member of the House Homeland Security Committee, told CNN. A spokeswoman for police at the Schiphol airport in Amsterdam declined comment about the case or about security procedures at the airport for Flight 253.

Dutch airline KLM says the connection in Amsterdam from Lagos, Nigeria, to Detroit involves a change in carrier and a change in aircraft.

Schiphol airport, one of Europe's busiest with a heavy load of transit passengers from Africa and Asia to North America, strictly enforces European security regulations including only allowing small amounts of liquid in hand luggage that must be placed inside clear plastic bags.

A spokesman for the Federal Airports Authority of Nigeria, Akin Olukunle, said all passengers and their luggage are screened before boarding international flights. He also said the airport in Lagos cleared a U.S. Transportation Security Administration audit in November.

"We had a pass mark," Olukunle said. "We actually are up to standards in all senses."

Nigeria's information minister, Dora Akunyili, condemned the attempted bombing. She said the government has opened its own investigation into the suspect and will work with U.S. authorities.

"We state very clearly that as a nation we abhor all forms of violence," Akunyili said in a statement issued Saturday.

London's Metropolitan Police also was working with U.S. officials, said a spokeswoman who spoke on condition of anonymity in line with department policy. A search was under way Saturday at an apartment building where Mutallab is said to have lived in a posh West London neighborhood.

University College London issued a statement saying a student named Umar Farouk Abdulmutallab studied mechanical engineering there between September 2005 and June 2008. But the college said it wasn't certain the student was the same person who was on the plane.

Delta Air Lines Inc., which acquired Northwest last year, said a passenger caused a disturbance, was subdued, and the crew requested that law enforcement officials meet the flight.

Passenger Syed Jafri, a U.S. citizen who had flown from the United Arab Emirates, said the incident occurred during the plane's descent. Jafri said he was seated three rows behind the passenger and said he saw a glow, and noticed a smoke smell. Then, he said, "a young man behind me jumped on him."

"Next thing you know, there was a lot of panic," he said.

Federal officials said there would be heightened security for both domestic and international flights at airports across the country, but the intensified levels would likely be "layered," differing from location to location depending on alerts, security concerns and other factors.

Passengers can expect to see heightened screening, more bomb-sniffing dog and officer units and behavioral-detection specialists at some airports, but there will also be unspecified less visible precautions as well, officials said.

Sen. Jay Rockefeller, D-W.Va., the chairman of the Senate Commerce Committee, said in a statement he would hold hearings in January to look into the incident and related security issues.

The FBI and the Homeland Security Department issued an intelligence note on Nov. 20 about the threat picture for the holiday season, which was obtained by The Associated Press. At the time, officials said they had no specific information about attack plans by al-Qaida or other terrorist groups.

President Barack Obama was notified of the incident and discussed it with security officials, the White House said. Officials said he is monitoring the situation and receiving regular updates from his vacation spot in Hawaii.

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Associated Press Writers Lara Jakes in Baghdad, Iraq, Jon Gambrell in Lagos, Nigeria, Arthur Max in Amsterdam, Jennifer Quinn in London, Ahmed al-Haj in Yemen, and Larry Margasak and Devlin Barrett in Washington contributed to this report.

PETN [Pentaerythritol tetranitrate]

Pentaerythritol tetranitrate, C5H8N4012 (PETN), has a specific gravity of solids of 1.76 and a confined detonation velocity of over 25,000 fps. PETN is used as a priming composition in detonators, a base charge in blasting caps, and a core load for detonating fuse. PETN is very much used in Detonating Cord of which it is the explosive core (Primacord), where it develops a velocity rate of 21,000 feet per second. Detonating cord is insensitive to friction and ordinary shock, but may be exploded by rifle fire. It also detonates sympathetically with the detonation of an adjacent high explosive.

PETN is one of the strongest known high explosives with a relative effectiveness factor (R.E. factor) of 1.66. It is more sensitive to shock or friction than TNT or tetryl, and it is never used alone as a booster. It is primarily used in booster and bursting charges of small caliber ammunition, in upper charges of detonators in some land mines and shells, and as the explosive core of primacord.

During World War II the M9A1 2.36" Rocket Launcher (Bazooka) charge, with 8 oz of pentolite, could penetrate up to 5 inches of armor.

Demolition charge, M118, commonly called Flex-X or sheet explosive, consists of 4 half-pound sheets of flexible explosive packed in a plastic envelope. Each sheet is approximately 3 inches wide, 12 inches long, and 1/4 inch thick. Note: The exact explosive contained in an M118 charge varies with the manufacturer. At present, some manufacturers use PETN as the basic explosive. Others use RDX. Charges manufactured in the future may include other explosives.

PETN does not occur naturally, so the production and use of this kind of compound can lead to contamination of the environment. PETN is subject to biodegradation in untreated or unpreserved urine and feces. There also have been some reports of its degradation by bacteria, whose PETN reductase sequentially denitrates PETN into tri- and dinitrates (French et al., 1996). The last compound shown in the pathway, pentaerythritol dinitrate, is degraded further to unknown products.

In 1995 Haustein KO, Winkler U, Loffler A, Huller G. of the Abteilung Klinische Pharmakologie, Medizinische Hochschule Erfurt, Germany reported on a study of PETN's cardiovascular effects. The effects of 80 mg pentaerithrityl-tetranitrate (PETN) as suspension or formulated as tablets were compared to placebo in a single blind, randomized, crossover study in 18 healthy subjects (study A), and the bioequivalence of two tablet formulations (marketed Dilcoran 80 vs a new formulation) was studied in 24 healthy subjects after administration of single oral doses of 80 mg PETN according to a placebo controlled, randomized, double blind, two-way crossover study design (study B). The perfusion of the right middle finger was measured by rheography (altitude A of the changes of resistance and of the incisure D) before and 24 h post-dose, and blood pressure and heart rate were measured in supine position at the same time. The values of area under curve (AUC) of the ratio A/D were calculated by the trapezoidal rule. In study A the mean A/D-values were reduced from about 2.0 to about 1.3 after intake of PETN (solution or tablet) with a minimum 60 to 90 min postdose (solution) and 2 h postdose (tablet). A significant reduction in this ratio was seen up to 8 (solution) or 12 h (tablet) post dose. Changes in blood pressure were not observed while the heart rate decreased in the subjects of all three groups 1 to 2 h postdose followed by an increase by 6 to 10 beats per min. After subtraction of the AUC values of placebo from the PETN-derived AUC values, mean values of 6.61 (SD 1.52, solution) and 7.25 (SD 1.48, A/D*h, tablet) were calculated (p > 0.1, study A).