LONDON, AP – The investigation into a plot to blow up jetliners over the Atlantic zeroed in Saturday on brothers arrested in Pakistan and Britain, one named as a key al-Qaida suspect who left the family’s home in England years ago and the other described as gentle and polite.

British authorities, meanwhile, warned against complacency, saying the detention of several dozen suspects had not eliminated the danger. The terror threat level in Britain remained “critical” — its highest designation — and delays, flight cancellations and intense security continued to greet travelers at London airports.

“No one should be under any illusion that the threat ended with the recent arrests. It didn’t,” Home Secretary John Reid told police chiefs at a breakfast meeting. “All of us know that this investigation hasn’t ended.”

Among the questions British police are studying is whether any of the suspects had links to last year’s London suicide bombers and how many visited Pakistan in recent months. They also are examining Internet cafes near the suspects’ homes, looking into the possibility of tracking Web based e-mails or instant messages, Scotland Yard said.

With U.S. authorities urgently investigating whether the British plotters had ties in America, a news report said at least one of the men under arrest in Britain had contact in Germany with the wife of Sept. 11 fugitive Said Bahaji. The report in Focus, a German weekly, did not specify the suspect involved or say when the contact occurred.

British investigators and officials have not said how close the plot was to fruition when the arrests were made, but U.S. officials have said they would not have likely waited as long.

In June, U.S. law enforcement officials arrested seven young men in Miami, claiming they’d plotted to bomb the Sears Tower in Chicago and a federal building in Miami.

“You want to go and disrupt cells like this before they acquire the means to accomplish their goals,” U.S. Attorney R. Alexander Acosta said.

One intelligence veteran suggested cultural and legal differences could account for why British authorities are more willing than their American counterparts to watch and wait before making a move in a terror case.

“It’s just the way they work,” said Stan Bedlington, of Arlington, Va., a former CIA senior terrorism analyst who also served in the Special Branch intelligence services of the British Colonial Police. “They (British) would always hope that they could turn somebody and use them to their advantage,” he said.

But he said he believed that the ability of the British to round up suspects without bringing formal charges helped them wait longer.

“In America, they’re very much afraid of an operation going down before they can stop it. It’s a matter of culture,” Bedlington said.

A swirl of attention has focused on the role that the brothers Rashid and Tayib Rauf may have played in the airliner plot. Their father, Abdul Rauf, immigrated to Britain from the Mirpur district of Pakistan several decades ago, and his five children were all born in Britain, the family said.

Rashid Rauf was arrested about a week ago along the Pakistan-Afghan border, and Pakistani officials have characterized him as a “key person” in the airline plot. They said evidence linked him to an “Afghanistan-based al-Qaida connection” but gave no details.

His 22-year-old brother, Tayib, was taken into custody in Britain during the sweeps that nabbed 24 people here, and unconfirmed reports said a third brother might have been detained.

A great-uncle of the Rauf brothers said Tayib is partially deaf due to a childhood illness.

“He is very, very polite, the kindest person you could hope to meet,” Qazi Amir Kulzum was quoted as saying in Saturday’s edition of the Birmingham Post. “No one can believe that he would be involved in such matters.”

Neighbors and friends of the Raufs expressed shock that the brothers were caught up in the inquiry, but the devout Muslim family is no stranger to authorities.

The Raufs’ terraced home was first searched during a 2002 investigation into the fatal stabbing of Mohammed Saeed, an uncle of the brothers, police said. Rashid Rauf was reportedly a suspect in the slaying and is thought to have left England for Pakistan shortly after the death.

The house was searched again in connection with a murder during race riots in 2005.

British authorities have released little information about the brothers, or the course of their investigation into the alleged terror plot in general. There were no briefings Saturday for the second straight day, and senior government figures stayed largely out of sight.

The British government warned news media not to put the investigation at risk by publishing details about the plot. Reid, the home secretary, and Attorney General Lord Goldsmith called for “considerable restraint” to avoid tainting any trials.

They said the government was trying to “strike the balance between the need to provide necessary information to the public and to business whilst avoiding prejudicing ongoing investigations or future proceedings.”

But in Islamabad, where authorities are eager to put a positive spin on a story that has again put Pakistan at the center of a major international terror investigation, officials spent Saturday leaking details of their country’s role in cracking the case.

Pakistan is questioning at least 17 people, including Rashid Rauf and one other British national whose name has not been released.

A senior Pakistani security official told The Associated Press that Rauf’s arrest prompted an accomplice in the southern city of Karachi to make a panicked phone call to a suspect in Britain, giving the green light for the airliner plot to move forward urgently.

“This telephone call intercept in Karachi and the arrest of Rashid Rauf helped a lot to foil the terror plan,” the official said.

A second intelligence official, who described the accomplice as “inexperienced,” also said the caller “alerted his associates about the arrest of Rashid Rauf, and asked them to go ahead.”

Both officials agreed to discuss the investigation only if not quoted by name due to the sensitive nature of their work.

While authorities in Pakistan believe they have nabbed the main players in the plot, the second intelligence official said two or three suspects remained at large, including Matiur Rahman, a senior figure in the al-Qaida-linked Pakistani militant group Lashkar-e-Jhangvi. He said Rahman’s name was mentioned by one of the detainees during interrogation.

British police on Friday released one of the 24 people originally arrested. No charges have been filed yet against the others. Under tough new anti-terrorism laws, authorities can hold suspects up to 28 days without charge, but pressure is likely to mount for police to disclose at least some of the evidence.

Many in Britain’s Muslim community are deeply distrustful of the police following high-profile blunders in the past, including the killing of a man mistaken for a suicide bomber and the shooting of another man in a raid that resulted in no charges.

Prominent British Muslims, including three members of Parliament, complained in an open letter Saturday that Britain’s intervention in Iraq and the failure to secure an immediate cease-fire in Lebanon and to get Israel to release jailed militants provided “ammunition to extremists who threaten us all.”

India’s government, meanwhile, banned liquids from airliners and limited carry-on luggage while intensifying security at airports and other public places Saturday.

The move came a day after the U.S. Embassy sent an e-mail to Americans in India warning that foreign militants, possibly al-Qaida terrorists, could be planning bomb attacks there. India has been the target of terror attacks by extremists linked to Islamic insurgents in Kashmir.