Mathew Martoma's insider trading trial took a surprising turn Thursday afternoon.
During the second day of his cross-examination, Dr. Sidney Gilman, the government's key witness in the trial of the former SAC Capital portfolio manager, testified that the FBI told him its main target was Steven A. Cohen.
Gilman, a retired neurology professor from the University of Michigan, testified that when he was first approached by the FBI on Sept. 1, 2011, the agents interviewing him told him he "was only a grain of sand, as is Mr. Martoma," and that "they were really after a man named Steven A. Cohen."
Gilman received a nonprosecution agreement in return for his testimony. Under the deal, he is required to tell the truth about all events while on the stand or the agreement could be voided.
SAC Capital pleaded guilty in November to insider trading charges and paid the government a record $1.8 billion to settle the charges.
(Read more: Martoma pleads not guilty)
Gilman will continue his testimony Friday morning at the federal courthouse in lower Manhattan.