The US Attorney's office is investigating Bill Clinton's last-minute decision to commute the sentences of four men convicted of stealing millions in government funds.
The investigation is expected to examine whether Clinton pardoned the men in exchange for votes from their Hasidic Jewish community for his wife's Senate campaign last autumn.
The community of New Square, in Rockland County, New York, voted overwhelmingly in favour of Hillary Clinton when she won election to the Senate in November.
Manhattan US Attorney Mary Jo White and the FBI, in a joint statement last week, announced they were opening an investigation into presidential pardons but declined to provide any details.
The investigation also is said to include the pardon of fugitive financier Marc Rich, whose former wife, Denise, is a major Democratic fund-raiser.
Mrs Clinton has said that she sat in on a December meeting with supporters of clemency for the four Hasidic men but said she played no part in her husband's decision to commute their sentences.
She has also said she had no conversations whatsoever about the commutations during pre-election meetings with Hasidic leaders.
The president and his wife, then New York's senator-elect, met for about 45 minutes on December 22 with two leaders from the New Square Hasidim who supported clemency for Kalmen Stern, David Goldstein, Benjamin Berger and Jacob Elbaum. The meeting took place in the White House Map Room.
Supporters of the four men have not denied their guilt but said they did not keep the money for themselves, and gave it instead to a school and residents of the village. The men are accused of created a fictitious religious school to receive government money.
It is unusual for a first lady to sit in on a clemency meeting but Mrs Clinton said that meeting was the first time they had spoken to her of the clemency.