Washington (CNN) -- The U.S. Marshals Service lost track of two former participants in the federal Witness Security Program "identified as known or suspected terrorists," according to the public summary of an interim Justice Department Inspector General's report obtained by CNN.
The Marshals Service has concluded that "one individual was and the other individual was believed to be residing outside of the United States," according to the summary.
A Justice Department official said in response to follow up questions about the matter by reporters on Thursday that the pair had left the program years ago and had been accounted for.
It was not clear when or for how long the Marshals Service lost track of them.
The report notes that while in the middle of an audit of the witness program, the inspector general notified the Justice Department of national security vulnerabilities.
The agency watchdog "developed the interim report to help ensure that the Justice Department promptly and sufficiently addressed the deficiencies we found."
After its audit, the inspector general's office reported that "the department did not definitively know how many known or suspected terrorists were admitted" into the witness program," among other "significant issues concerning national security."
As of March 2013, the Justice Department was reviewing more than 18,000 program case files to determine whether more known or suspected terrorists have been admitted, the summary notes. As such, the number of terrorists lost or unaccounted for "may not be complete and may continue to evolve."
The summary said that although the Marshals Service was giving known or suspected terrorists who participated in the witness program and their dependents new names and identity documents, the Justice Department "was not authorizing the disclosure to a federal center that operates a terrorist watch list that helps provide information to the Transportation Security Administration's "no-fly" database.
"Therefore it was possible for known or suspected terrorists to fly on commercial airplanes in or over the United States and evade one of the government's primary means of identifying and tracking terrorists' movements and actions," the summary said.
The inspector general's office notified the Justice Department of the problems and in the middle of remedying them, the Marshals Service discovered it could not account for the two missing people.
A Justice Department official who spoke to reporters about the matter said on Thursday that the agency knows the whereabouts of witness program participants.
"As for the two people you're talking about, we know they left the country years ago, they left the program years ago, they have been accounted for. There has been no information provided that they have ever returned to the United States," according to the official who spoke
The Justice Department said in an earlier statement that the FBI had "not identified a national security threat tied to the participation of terrorism-linked witnesses."
A Justice Department official also told CNN that the pair "completed their obligations to the U.S. government by cooperating against terrorists" and were not fugitives. The official added that "for obvious reasons, we cannot comment on the exact location of any current or former participant" in the program.
The development broke in the midst of a difficult week for the Obama administration, with revelations that the Justice Department secretly collected months of phone records for reporters and editors at The Associated Press; renewed criticism and accusations of a cover-up over the White House's response to the deadly attack on the U.S. mission in Libya, and revelations that the Internal Revenue Service targeted conservative groups applying for tax-exempt status.
President Barack Obama did not respond when asked at the White House on Thursday about the inspector general's report.
CNN's Paul Courson contributed to this report.