Undercover investigators managed to smuggle bomb-making materials into key US government buildings on 10 occasions last year, a congressional report says.
The Government Accountability Office said that once inside the buildings, investigators were able to assemble bombs and carry them around freely, BBC reported.
The infiltrated buildings included the offices of a senator and a congressman.
The report cited instances of guards falling asleep on the job or failing to detect explosives at x-ray scanners.
Washington spends around $1bn a year protecting thousands of government buildings across the country.
The BBC's Jon Donnison in Washington says the report makes embarrassing reading for the Federal Protection Service, which employs over 13,000 contract security guards.
The report flagged up an incident where one guard accidently fired his gun into a mirror in a public lavatory while practicing pulling it out of its holster.
On another occasion, a security officer did not notice that he had sent a baby through an X-ray machine.
The Senate Homeland Security Committee is due to hold a hearing into failings at the agency.
The committee chairman, Senator Joe Lieberman, said that eight years after the 9/11 attacks, it was "simply unacceptable" that federal employees and visitors remained exposed to potential terrorist attacks.