The government’s disclosure of a plot to assassinate the Saudi ambassador to the United States last month came with some reassuring news: the suspect, an Iranian-American, was in custody, had confessed and had provided valuable intelligence on Iran’s role in supporting the plan.

But unlike many confessions, these statements did not come in the hours after the arrest, or in the presence of a lawyer.

The purported cooperation by the defendant, Mansour J. Arbabsiar, occurred during the 12 days he was in custody after his arrest on Sept. 29. Prosecutors said that in the days after his arrest, Mr. Arbabsiar had “knowingly and voluntarily” waived his rights to remain silent, to have a lawyer present during his interrogation and to be quickly taken before a judge.

But Mr. Arbabsiar’s lawyer, Sabrina Shroff, said in a recent interview that she intended to seek a hearing on whether the “consent was freely given, or whether it was unlawfully extracted,” given the gap in time between her client’s arrest and his initial court appearance in New York on Oct. 11. “There has to be a deep concern about the voluntariness of consent to that long a period of detention,” she said.