ALEXANDRIA, Va. – The FBI agent who arrested Zacarias Moussaoui in August 2001 accused headquarters of criminal negligence for its refusal to investigate Moussaoui aggressively after his arrest, according to court testimony yesterday.

Agent Harry Samit testified under cross-examination at Moussaoui’s trial that FBI headquarters’ refusal to follow up “prevented a serious opportunity to stop the 9/11 attacks” that killed nearly 3,000 people.

Moussaoui is the only person charged in this country in the attacks.

The FBI’s actions between Moussaoui’s arrest on immigration violations on August 16, 2001, and September 11, 2001, are crucial to his trial because prosecutors allege that Moussaoui’s lies prevented the FBI from thwarting or at least minimizing the September 11 attacks. Prosecutors must prove that Moussaoui’s actions caused the death of at least one person on September 11 to obtain a death penalty.

The defense argues that nothing Moussaoui said after his arrest would have made any difference to the FBI because its bureaucratic intransigence rendered it incapable of reacting swiftly to Moussaoui’s arrest under any circumstances.

Under cross-examination by defense attorney Edward MacMahon, Mr. Samit acknowledged that he predicted in an August 18, 2001, memo that Moussaoui was a radical Islamic terrorist in a criminal conspiracy to hijack aircraft. Moussaoui ended up pleading guilty to two specific counts that Mr. Samit had explicitly predicted in his August 18 memo.

Despite Mr. Samit’s urgent pleadings, FBI headquarters refused to open a criminal investigation and refused Mr. Samit’s entreaties to obtain a search warrant.

“You needed people in Washington to help you out?” Mr. MacMahon asked.

“Yes,” Mr. Samit said.

“They didn’t do that, did they?”

Mr. Samit said no.

He confirmed under questioning that he had attributed FBI inaction to “obstructionism, criminal negligence and careerism” in an earlier report.

One FBI supervisor in Washington told Mr. Samit that he was getting unnecessarily “spun up” about his concerns over Moussaoui.

Moussaoui pleaded guilty in April to conspiring with Al Qaeda to hijack aircraft and commit other crimes. The sentencing trial will determine his punishment: death or life in prison.

Moussaoui denies he had anything to do with September 11 and says he was training for a future attack.

Mr. MacMahon also questioned Mr. Samit on whether the government could have legally searched Moussaoui’s Minnesota hotel room without first obtaining a warrant.

Mr. Samit said that in certain circumstances agents can conduct a search on foreign nationals immediately and obtain a warrant after the fact. But he said in Moussaoui’s case, he and his supervisors determined that it would be best to arrest Moussaoui first.

Mr. Samit testified before the trial’s recess last week that Moussaoui lied to him after his arrest and thwarted his ability to obtain a search warrant. Mr. Samit said that the FBI would have launched an all-out investigation if it had been able to search Moussaoui’s belongings.

“You blew an opportunity to search … without arresting him?” Mr. MacMahon asked Mr. Samit.

Mr. Samit responded, “That’s totally false.”

He said he found himself in a bureaucratic bind because he had opened an intelligence investigation on Moussaoui rather than a criminal investigation and therefore needed Justice Department approval to get a search warrant. Many of the barriers between criminal and intelligence investigations were removed after September 11.