Here's a friendly reminder from the Pentagon police chief: please fill out the proper forms before you bring a bomb to work.

The advice came after security guards at the Defense Department's entrance discovered parts of a disabled homemade bomb on a civilian entering the building last month, officials said.

The incident may have set off alarm bells at other government offices but police at the sprawling headquarters for the US military are accustomed to unusual items carried in by employees.

In this case the civilian had brought in the disarmed improvised explosive device (IED) to brief senior naval officers on roadside bombs -- the number one killer of American troops over the past decade.

"He had inert components of an IED, basically wires and switches," said Pentagon spokesman Lieutenant Colonel Tom Crosson.

But there was "nothing explosive," he told AFP.

The civilian, employed by the US Navy, had failed to secure written permission before showing up on November 19.

After some phone calls and checks, he was eventually let in with the bomb parts, Crosson said.

Steven Calvery, head of the Pentagon Force Protection Agency, which handles security for the giant office building in Arlington, Virginia, has since asked employees to brush up on procedures for bringing in inoperable guns or disarmed explosives.

He wrote in a memo that it would be "helpful" to ensure workers are familiar with the "authorization process" as these items require written approval from his force, according to a defense official, who spoke on condition of anonymity.

In a surprise security check last month in which employees and visitors were randomly searched and put through x-ray screening, the Pentagon police confiscated 21 knives, seven pepper spray canisters, five shotgun shells, and a baton, said the official.

The searches led to the arrest of a civilian working for the US Army for carrying illegal drugs.

But as long as the right papers are filled out and signed by the right people, employees can bring in firearms for official events, the defense official said.

"It's not unusual for people to bring in guns and rifles that are used in ceremonies...or presented as gifts," the official said.

"There's a procedure. There's a process to get them into the building."