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A convicted felon and former member of a domestic terrorist group was allowed to use expedited airport security lines last summer, according to the Department of Homeland Security's internal watchdog.

The DHS Inspector General's Office said Thursday a whistle-blower flagged the incident as a "significant aviation security breach," prompting an investigation.

It issued a report saying that the traveler — who had convictions for murder and explosives offenses — had not applied for the Transportation Security Administration's PreCheck program but was selected for expedited security screening through the "risk assessment rules" in the Secure Flight program.

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A security officer at the airport recognized the felon and alerted his supervisor, but was told to "take no action" and allow the passenger through the TSA PreCheck lane, the Inspector General's Office's report said.

"Mitigating and reducing passenger screening vulnerabilities is important to our nation's aviation security," Inspector General John Roth said in a statement. "Incidents like this highlight the need for TSA to modify their PreCheck procedures."

TSA Precheck allows passengers who have been pre-approved to keep on their shoes and belt, not remove their jackets, keep their laptops inside their cases, and not have to remove select liquids and gels from their bags.

The Inspector's General office said it had made two recommendations to prevent such incidents in the future and issued a classified report to the TSA regarding the need to modify PreCheck vetting and screening processes.

The TSA said in a statement that it "takes its responsibility for protecting the traveling public very seriously" and continues to enhance its "layered security approach."

All passengers, including those with TSA PreCheck marked on their boarding passes, are subject to "robust security," it said.

— Cassandra Vinograd