A Defense Department investigation has failed to identify the individuals who leaked information to The Washington Post about a draft report on the potential impact of allowing gays to serve openly in the military.

Defense Secretary Robert M. Gates ordered an investigation into the leak in November shortly after The Post published a front-page story saying that the report had concluded that the military could lift the ban on gays serving openly in uniform with only minimal and isolated incidents of risk.

The report had been closely guarded, and officials condemned what they described as the premature release of its findings.

Officials with the Defense Department’s Inspector General interviewed 101 people with access to the draft report — and reviewed 55,000 e-mails and 1,500 phone and wireless handheld records — but were unable to identify the two anonymous sources who spoke with The Post, according to a copy of the investigative report.

Investigators said they “could not exclude the possibility that persons outside DoD” provided information to The Post, and noted that they did not interview five White House staff members given access to the draft report.

The investigative report, which included e-mails from some of The Post’s reporters, was submitted to the Defense Department’s inspector general in April but leaked only this week to the Center for Military Readiness, a nonprofit group whose president is a leading critic of ending the ban on gay men and women serving openly in the military.

The policy will officially end 60 days after President Obama certifies — in consultation with the defense secretary and Joint Chiefs of Staff Chairman Adm. Mike Mullen — that the military is ready to lift the ban. Though gay rights activists had hoped the ban would end before Gates departs this week, the defense secretary has said in recent interviews that any final decision is likely to be handled by his successor, Leon Panetta.