MIAMI — The FBI initially sought terror suspect Jose Padilla’s cooperation to help prevent what intelligence sources indicated might be an imminent Al Qaeda attack, an FBI agent testified yesterday.

Padilla refused and was later implicated in an alleged radioactive “dirty bomb” plot and declared an enemy combatant.

Russell Fincher, an agent in the FBI’s New York-based unit investigating Osama bin Laden, testified at a pretrial hearing for Padilla that he met him at Chicago’s O’Hare International Airport in May 2002 believing he could gain information on Al Qaeda’s plans.

“I believed there was a terrorist act that was going to happen. I believed he had knowledge of that. I needed his help,” Mr. Fincher said of Padilla. “I didn’t want to arrest him.”

Mr. Fincher said that during a five-hour interview Padilla talked freely about his criminal past as a Chicago gang member, his conversion to Islam, and his travels in Egypt, Pakistan, and elsewhere. But when his story didn’t add up, Mr. Fincher said he confronted Padilla with terrorist allegations and asked if he would testify before a grand jury about his purported Al Qaeda connections.

“He stood up and told me the interview was over, and it was time for him to go,” Mr. Fincher said.

Padilla was arrested on a material witness warrant, which allows a person with direct knowledge of alleged criminal acts to be taken into custody. A month later, President Bush designated him an “enemy combatant” and he was placed in military custody. He remained there until November, when he was charged in an existing Miami case.

The charges in the Miami case do not mention the “dirty bomb” plot but do claim that Padilla trained at an Al Qaeda camp in Afghanistan. Padilla has pleaded not guilty.

The hearing yesterday concerned whether Padilla was officially in law enforcement custody when the FBI agent interview him and, if so, whether he was properly advised of his rights. Padilla’s lawyers said his statements should be barred from trial if those rights were violated. U.S. Magistrate Judge Stephen Brown did not indicate when he would rule.

Mr. Fincher testified that Padilla wasn’t read his Miranda rights until the interview ended. But he also said that Padilla agreed repeatedly to talk with the FBI agents, was never handcuffed or otherwise restrained and never requested a lawyer.

The initial pretext for Padilla’s airport interview was the discovery by customs officials that he was carrying more than $10,000 in cash but had declared only $8,000.

Although he did ask three times to call his mother in Florida, each time he changed his mind, Mr. Fincher said. When the FBI offered to contact her for him, Mr. Fincher said, Padilla declined because he didn’t want her to think he was in trouble again.

The FBI also offered to put Padilla up in a hotel to rest so the interview could resume the next day, but again Padilla refused, saying he wanted to clear up the issue about the cash. “He didn’t believe it was a big deal,” Mr. Fincher said.

Padilla is scheduled to go to trial in September along with two others on charges of participating in a North American network that supported Islamic extremists worldwide.