Sen. Tom Coburn (R-Okla.) said Friday that he has provided information to federal authorities investigating whether Sen. John Ensign (R-Nev.) broke the law in trying to keep secret his affair with a part-time staff member.

Before Ensign publicly admitted the adulterous relationship last June, Coburn had been a key behind-the-scenes counselor to Ensign. Coburn had urged his friend to end the affair with Cynthia Hampton and later tried to help him mediate the tension when her husband, a senior aide to the Nevada senator, confronted Ensign about the affair in late 2008.

Investigators are looking into whether Ensign then tried to help Doug Hampton get lobbying work, through meetings with key donors and administration officials. Such actions could violate federal laws and congressional rules that require departing congressional staff members to avoid lobbying for a year. The Justice Department has issued subpoenas seeking information to more than five Las Vegas companies tied to Ensign.

Ensign's parents also made $96,000 in payments to the Hamptons, funds the senator may have been required to report under federal disclosure laws as part of a severance package.

On Friday, Coburn confirmed that the Justice Department had requested copies of particular e-mail correspondence and said he was voluntarily cooperating with the probe and was not served with a subpoena. Coburn did not disclose the nature of the correspondence but said only a small number of his e-mails met the prosecutors' specific request.

"Dr. Coburn has also said he will gladly cooperate with any inquiry into the matter," Coburn spokesman John Hart said. "He went above and beyond DOJ's request."

Ensign's attorney, Robert Walker, declined to comment on the development.

When asked last year about the affair, Coburn originally denied knowing about it, then he balked at discussing his conversations with Ensign by asserting they were "privileged," due to his position as an ordained deacon and a doctor. (Coburn is an obstetrician.) He later retreated from that position.

Coburn told ABC News in November that he did nothing wrong in trying to mediate.

"Look, my whole goal in this thing was to bring two families to a closure of a very painful episode," he said.

"And there's no question that Doug called me and said, 'Will you talk to John about solving a problem?' And so I called John Ensign and said, 'Do you want me to talk to him?' [Ensign] said, 'Yes.' "