ROMULUS, Mich. - A Nigerian man who said he was an agent for al-Qaeda tried to blow up a Northwest Airlines plane yesterday as it was preparing to land in Detroit, but travelers who smelled smoke and heard what sounded like firecrackers rushed to subdue him, the passengers and federal officials said.

Northwest Flight 253 with 278 passengers and 11 crew members aboard was 20 minutes from Detroit Metro Airport when it sounded as if a firecracker had exploded, witnesses said. One passenger jumped over others and tried to subdue the man. Shortly afterward, the suspect was taken to a front-row seat with his pants cut off and his legs burned, a passenger said.

The White House said it believed the incident was an attempted act of terrorism. Stricter security measures were quickly being imposed on air travel, officials said, but they did not specify what those were.

Law enforcement officials identified the suspect as Umar Farouk Abdul Mutallab, 23. Others provided slightly different variations of his name.

One law enforcement source said the man claimed to have been instructed by al-Qaeda to detonate the Airbus 330-300, which was coming in from Amsterdam, over U.S. soil.

A federal counterterrorism official who asked not to be identified said that the suspect was apparently in a government law enforcement-intelligence database but that it was not clear what extremist group or individuals he was linked to, the New York Times reported last night.

He was not on the Transportation Security Administration's "no-fly" list, authorities said.

The man was being questioned yesterday evening. An intelligence source said he was being held and treated at an Ann Arbor, Mich., hospital.

Sources told CNN that the suspect had flown into Schiphol Airport in Amsterdam on a KLM flight from Lagos, Nigeria. A spokeswoman for police at Schiphol declined to comment about the case or about security procedures at the airport for Flight 253. Security at Nigeria's two major international airports remains a point of concern.

Peter Smith, a passenger from the Netherlands, said the setting off of the device "sounded like a firecracker in a pillowcase. First there was a pop, and then [there] was smoke."

At least one passenger acted heroically. Smith said that passenger, sitting opposite the suspect, climbed over other passengers, went across the aisle, and tried to restrain the man. The passenger who acted appeared to have been burned.

Michigan native Melinda Dennis was sitting in first class when the suspect was placed in a seat across the aisle from her.

"He didn't say anything," said Dennis, who lives in Europe and was connecting to a flight to Arizona. "He was burned very severely on his leg.. . . He was very calm and didn't show any reaction to pain."

The incident was reminiscent of one involving convicted shoe bomber Richard Reid, who tried to destroy a trans-Atlantic American Airlines flight in 2001 with explosives hidden in his shoes but was subdued by other passengers. Reid is serving a life sentence.

Delta Air Lines, the parent company to Northwest, said in a statement: "Upon approach to Detroit, a passenger caused a disturbance onboard Northwest Airlines Flight 253. The passenger was subdued immediately and the crew requested that law enforcement meet the flight upon arrival."

Federal authorities have been told that the suspect allegedly had taped some material to his leg, then used a syringe to mix some chemicals with a powder while on the plane, one official said.

Officials described the device as incendiary rather than explosive, pending tests by forensics experts at the FBI. Incendiary devices generally deliver less of an impact than explosive devices.

CNN reported that, according to sources, the remains of the device were being sent to an FBI explosives lab in Quantico, Va., for analysis.

The FBI and the Homeland Security Department issued an intelligence note Nov. 20 about the threat picture for the 2009 holiday season. At the time, intelligence officials said they had no specific information about attack plans by al-Qaeda or other terrorist groups. The intelligence note was obtained by the Associated Press.

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

The Homeland Security Department said late yesterday that passengers "may notice additional screening measures, put into place to ensure the safety of the traveling public on domestic and international flights."

It said it encouraged "those with future travel plans to stay in touch with their airline and to visit www.tsa.gov for updates."

Nothing seemed out of the ordinary about Flight 253 until it was on its final approach to Detroit, said a Federal Aviation Administration spokeswoman, Elizabeth Isham Cory. Then, the pilot declared an emergency and landed without incident shortly thereafter, Cory said. The plane landed at 12:51 p.m. EST.

Syed Jafry of Holland, Ohio, a U.S. citizen aboard the flight who had flown from the United Arab Emirates, said the incident occurred during the plane's descent. Jafry said he was seated three rows behind the suspect and 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.

"Smoke, flames, yelling, and screaming" are what Iliana Schilke of Livonia, Mich., remembered, sitting a couple of rows behind the suspect.

"It was a fire. It wasn't just a firecracker," said Calvin Kakar of New York, who was a few rows in front of the suspect.

Passengers yelled for water, flight attendants ran to get the fire extinguisher, and the fire was soon doused, witnesses said.

More than one person aboard the plane reported having seen another passenger taping the incident with a video camera.

Donald Dawkins, a spokesman with the Bureau of Alcohol, Tobacco, Firearms and Explosives in Detroit, said the bureau's K-9 unit searched the bags aboard the plane. No other explosives or contraband were found in the suspect's luggage or in any other passenger's.

This article includes information from the Washington Post and the Detroit Free Press.