WASHINGTON - U.S. authorities overseas have interrogated a second suspect in the alleged al-Qaida plot to detonate a radioactive bomb in America, officials said yesterday, as investigators scrambled to determine if other accomplices are in the United States, Switzerland, Egypt or elsewhere.

U.S. officials also said the so-called "dirty bomb" plan apparently called for stealing radioactive material from an unidentified American university laboratory. Low-level nuclear isotopes are widely used in medicine, research and other fields.

The plot, which was still in its early stages, was foiled when CIA, FBI, Customs and State Department agents identified and tracked Jose Padilla - a Brooklyn-born Muslim convert who adopted the name Abdullah al Muhajir - in Cairo, Egypt, and Zurich, Switzerland. He was arrested May 8 when he flew from Switzerland to Chicago on what officials called a scouting mission for a terrorist attack.

President Bush said Padilla was one of many "would-be killers" that the U.S. has captured and that it is looking for many more.

"This guy Padilla's one of many who we've arrested," Bush said in a meeting in his Cabinet Room. "The coalition we've put together has hauled in 2,400 people. And you can call it 2,401 now. There's just a full-scale manhunt on. ... We will run down every lead, every hint. This guy Padilla's a bad guy, and he is where he needs to be: detained."

Officials said Padilla has refused to cooperate since his arrest. After Bush decided Sunday that Padilla should be held as an "enemy combatant" against the United States, rather than as a criminal defendant, the suspect was flown by military C-130 aircraft to a high-security Navy brig outside Charleston, S.C., where he has been isolated from other inmates and is under heavy military guard.

Yesterday, Padilla's lawyer told a federal court hearing in New York that Padilla's continued detention was a violation of the Constitution because he had not been charged and was being denied access to legal counsel. "My client is a citizen," attorney Donna R. Newman told reporters. "He still has constitutional rights."

A senior U.S. intelligence official said Pakistan detained a second suspect in the plot last month. The official said the man, who has not been publicly identified but is from an Arab country in the Middle East, is being interrogated by U.S. authorities at an undisclosed location. There were conflicting reports as to whether Pakistan had handed over the suspect to U.S. authorities.

The second suspect traveled with Padilla to eastern Afghanistan last fall to meet Abu Zubaydah, al-Qaida's operations chief, and later accompanied Padilla to secret meetings with other senior al-Qaida leaders inside Pakistan to discuss the "dirty bomb" proposal as well as potential attacks against hotels, gas stations and other targets, the official said.

One of the most urgent aspects of the investigation is whether Padilla had other accomplices, particularly in the United States.

"He clearly had associates, and one of the things we want to ask him about is who those associates were and how we can track them down," Deputy Defense Secretary Paul D. Wolfowitz said on CBS's Early Show.

U.S. intelligence officials said they had not determined whether Padilla was a seasoned al-Qaida operative who had escaped detection until recently, or was a free-lance agent who somehow made contact with senior al-Qaida leaders last fall and was embraced by the group.