Federal investigators focused Tuesday on the possible involvement of the Pakistani Taliban in the failed Times Square bombing as they pieced together clues and charged a suspect who was pulled off an airplane as he headed to his native Pakistan, according to court documents and law enforcement sources.

Faisal Shahzad, a 30-year-old U.S. citizen arrested late Monday at John F. Kennedy International Airport, admitted his involvement in the plot, authorities said, and told FBI agents he received bombmaking training in a region of Pakistan known as a militant hotbed. Shahzad, who became a naturalized citizen last year, is from a military family in Pakistan, where he spent five months before returning in February to his home in a leafy, quiet neighborhood of Shelton, Conn.

His reported confession, combined with a series of phone calls he received from Pakistan after purchasing the Nissan Pathfinder used in the attempted bombing, has led investigators to zero in on the Pakistani Taliban connection as "a leading theory," a federal law enforcement official said.

"It's a leading line of inquiry," said the official, who spoke on the condition of anonymity because the international probe is in its early stages. "There are only a few organizations in Pakistan that could provide training, and the Pakistani Taliban is . . . one that has an ax to grind with us." Pakistani Taliban claims of responsibility for Saturday night's attempt, which investigators had played down, are being reevaluated, said the official, who added that al-Qaeda involvement "is a leap at this point."

The focus on a group that had been considered uninterested in launching attacks outside Pakistan or Afghanistan pointed up the gravity of an incident that authorities characterized as a potentially deadly strike against the United States, albeit with an unsophisticated homemade device that failed to detonate. Even as officials praised the rapid law enforcement response, the incident resurrected the controversy over the Obama administration's handling of the attempted bombing of a Detroit-bound airliner on Christmas Day. Critics said the suspect in that case should have been placed in a military, rather than civilian, court.

Speaking at a news conference at which the government announced five felony counts against Shahzad, Attorney General Eric H. Holder Jr. said, "It is clear that this was a terrorist plot aimed at murdering Americans in one of the busiest places in our country." Shahzad was charged with attempting to detonate the sport-utility vehicle that was set ablaze on a tourist-crowded block in Midtown Manhattan and trying to kill bystanders and destroy property.

The charges in federal court in Manhattan came on a day of familiar political and law enforcement rituals. Obama administration officials, seeking to navigate the perilous politics of terrorism, detailed their intensive involvement in Shahzad's apprehension and emphasized that he was providing useful intelligence to authorities. Some Republicans questioned whether key clues had been missed.

"Like the Christmas Day bomber, we were lucky that both of these folks were incompetent -- they couldn't trigger the explosives," said Sen. Christopher S. Bond (Mo.), the ranking Republican on the Senate intelligence committee.

Multiple U.S. law enforcement officials said Shahzad had attracted no significant law enforcement attention before the attempted bombing. "He was not on the radar," one official said.

Also triggering debate was the decision to read Shahzad his "Miranda" rights against self-incrimination. The Miranda issue rose to prominence after the Nigerian suspect in the Christmas Day incident, Omar Farouk Abdulmutallab, stopped cooperating with authorities after being read his rights. Some Republicans, including Sen. John McCain (R-Ariz.), said Shahzad should not have been afforded that constitutional right "until we find out what it's all about."

But administration officials said Shahzad, who, like Abdulmutallab, was initially responsive to questioning under a "public safety exception" to the Miranda rule, continued to cooperate after his rights were read to him. They also pointed out that Shahzad is a U.S. citizen and must be tried in civilian, not military, court.

Officials canceled Shahzad's scheduled appearance in Manhattan federal court amid his reported dialogue with agents. It was unclear whether a lawyer had been appointed for him; he is scheduled to face a judge on Thursday at the earliest.

The Times Square incident was the latest in a series of attempted terrorist attacks against the United States over the past year. As with some other suspects, it was difficult to reconcile the grim portrait of Shahzad painted by authorities with the accounts of those who know him. Shahzad's neighbors described him as a pleasant family man who enjoyed taking care of his yard and playing with his two daughters.

U.S. investigators found Shahzad after a two-day investigation combining old-fashioned shoe-leather detective work, sophisticated searches through telephone and electronic records, and the latest linkups among federal immigration, travel and border databases.

Authorities said they had identified Shahzad by Sunday night as " "a person that we would like to talk to," FBI Deputy Director John S. Pistole said. Tracing the origins of the Pathfinder was a crucial part of the probe. The car's vehicle identification number had been removed from a dashboard plate. But authorities were able to find the seller by using a decal on the tailgate to trace the car to a Connecticut used-car dealer, who early Sunday gave them sales records on two cars matching the Pathfinder's description.

Authorities then found the vehicle's registered owner in Connecticut. Police officials said that was a major break in the case. Another key step was finding an e-mail from Shahzad to the car's seller. Although Shahzad used a fake name, he included a number from a disposable cellphone, which investigators used to determine his identity.

Investigators served a search warrant Tuesday at Shahzad's home and visited a gun shop in Shelton where he bought a 9mm Kel-Tec rifle in March. Court documents said investigators found an unspecified gun in a car that Shahzad left at the airport.

The probe also extended to Pakistan, where officials said FBI agents were expected to push their Pakistani counterparts for access to intelligence about the Pakistani Taliban and its possible involvement in the plot. Pakistani officials pledged cooperation.

A Pakistani intelligence official said Tuesday that authorities had arrested at least two people in the southern port city of Karachi in connection with the Times Square plot. But a U.S. law enforcement official said the arrest was "not at our behest."

The Pakistani Taliban has waged a campaign of bombings and assassinations against the Pakistani government in recent years. Until now, U.S. and Pakistani intelligence officers didn't think the group had the reach necessary to execute attacks outside its home region, and it has traditionally shied away from the sort of global jihad espoused by al-Qaeda.

But the group's ties with al-Qaeda and other foreign militants have expanded of late, and so have its ambitions. In a video issued this week, Pakistani Taliban leader Hakimullah Mehsud promised more strikes against the United States, and said that suicide bombers had infiltrated American cities.