The CIA said on Thursday it had opened an "exploratory" investigation into Petraeus' conduct, building on the FBI's probe.

"If there are lessons to be learned from this case, we'll use them to improve," said CIA spokesman, Preston Golson.

"But we're not getting ahead of ourselves. An investigation is exploratory and doesn't presuppose any particular outcome," he added.

Petraeus, widely regarded as one of the most respected public servants in the US, resigned last week over an extramarital affair with his biographer, Paula Broadwell.

After a search, Broadwell was found to have classified documents in her possession, but both she and Petraeus have denied that those documents had come from him. It's still unclear what kind of information those documents contain.

US Attorney General Eric Holder said so far, the FBI had not found any "national security threat" in the course of its probe.

Petraeus will testify before both the House and Senate intelligence committees on Friday as they probe the September 11 attack on the consulate in Benghazi, in which the US ambassador to Libya, Chris Stevens, and three more Americans died.

hc/ccp (Reuters, AFP)