Sachtleben, who was a consultant to the FBI after retiring from the law enforcement agency in 2008, gave information about an "underwear bomb" the FBI analyzed as part of its investigation into a failed al-Qaida scheme to blow up an airliner bound for the United States, The Wall Street Journal reported.
Sachtleben said he was "deeply sorry" for his actions, which federal authorities said was a serious national security leak, the newspaper said.
"While I never intended harm to the United States or to any individuals, I do not make excuses for myself," he said in a statement released through his attorneys. "I understand and accept that today's filings start the process of paying the full consequences of my misconduct, and I know that the justice system I once served so proudly will have its say."
The Associated Press wrote about the explosive device and the unsuccessful plot in May 2012.
The Journal said documents filed in U.S. District Court in Indianapolis Monday show Sachtleben has signed a plea agreement admitting to unauthorized disclosure of classified information and possessing child pornography.
He's expected to be sentenced to more than 11 years in prison, with most of the time due to the pornography charge.
An AP spokesman told the newspaper the news organization doesn't comment about its sources.