A senior US military official identified the second woman as Jill Kelley (37), who lives in Tampa, Florida, and serves as an unpaid social liaison to MacDill Air Force Base in Tampa, where the military's Central Command and Special Operations Command are located.
Kelley and her husband, Scott, said in a statement: "We and our family have been friends with Gen Petraeus and his family for over five years. We respect his and his family's privacy and want the same for us and our three children."
The anonymous military official said Kelley had received harassing emails from Broadwell, which led the FBI to examine her email account and eventually discover her relationship with Petraeus, who quit as CIA director on Friday.
The FBI contacted Petraeus and other intelligence officials, and Director of National Intelligence James Clapper asked Petraeus to resign.
A former associate of Petraeus confirmed the target of the emails was Kelley, but said there was no affair between the two.
The associate, who has been in touch with Petraeus since his resignation, said Kelley and her husband were longtime friends of Petraeus and wife, Holly.
Attempts to reach Kelley were not successful. Broadwell did not return phone calls or emails.
The Petraeus news caught much of Washington by surprise and members of Congress said yesterday that they want to know more details about the FBI investigation that revealed the extramarital affair between Petraeus and his biographer.
They questioned when the retired general popped up in the FBI inquiry, whether national security was compromised and why they weren't told sooner.
"We received no advanced notice. It was like a lightning bolt," said Democratic Sen Dianne Feinstein, who heads the Senate Intelligence Committee.
Petraeus (60) quit on Friday after acknowledging an extramarital relationship. He has been married for 38 years to Holly Petraeus, with whom he has two adult children, including a son who led an infantry platoon in Afghanistan as an army lieutenant.
Broadwell, a 40-year-old graduate of the US Military Academy and an Army Reserve officer, is married with two young sons. Petraeus' affair with Broadwell will be the subject of meetings on Wednesday involving congressional intelligence committee leaders, FBI deputy director Sean Joyce and CIA deputy director Michael Morell.
Petraeus had been scheduled to appear before congressional committees on Thursday to testify about the September 11 attack on the US Consulate in Benghazi, Libya, that killed four Americans.
Morell was expected to testify in place of Petraeus, and lawmakers said he should have the answers to their questions. But Feinstein and others didn't rule out the possibility that Congress will compel Petraeus to testify about Benghazi at a later date, even though he's relinquished his job.