Washington (CNN) -- A federal review cites poor judgment by a security guard who found a bag outside a government building in Detroit last year and stashed it under a desk where it remained for weeks -- with neither the guard nor anyone else in the building aware that it contained a bomb.
The report by the Homeland Security Department's office of inspector general released on Wednesday also said the guard's employer, a contractor, and the federal agency that oversees security at the Patrick V. McNamara Federal Building shared some responsibility as well for the bomb going undetected.
"Although the IED (improvised explosive device) did not explode, it represented a risk to the safety and security of the building and its occupants," the report said.
After the bomb was discovered it was detonated harmlessly by Detroit police. A Michigan man subsequently was charged with placing the device in a canvas bag outside the building.
The canvas bag, containing a small locked Sentry safe, was discovered February 26, 2011, outside the 27-story building that houses the FBI and other offices and remained at the guard desk until it was identified as a threat three weeks later.
During that time, at least two employees X-rayed it in an unsuccessful attempt to determine its contents. One guard shook it in another failed attempt to learn more, and a federal inspector conducted four routine checks of the guard post without discovering that the bag contained an explosive device, according to the report by the inspector general, an independent watchdog.
Rep. Bennie Thompson of Mississippi, the top Democrat on the Republican-led House Homeland Security Committee, asked for the audit to determine whether the guard's actions violated the private security company's contract with the Federal Protective Service (FPS), the government entity that protects federal property.
The assistant inspector general for inspections, Carlton Mann, concluded the contractor, DECO Inc., "committed multiple breaches of its contract" with the Federal Protective Service. But the report found that the shortcomings "were the result of poor judgment by the guard, not systemic problems with DECO."
Company officials had reason to believe the guard, who was not identified, "should have known how to respond to the bag," the report said.
In addition to receiving refresher training just three months before the incident, the guard was a sergeant with the Detroit Police Department. One FPS official asserted that he was one of the best guards in the building, the report said.
But on the day of the incident, the guard "misidentified the bag as found property and never treated it as suspicious, despite not knowing its contents," the report said.
The bag was placed under the security desk in the McNamara building, where other security guards did not consider it suspicious because the space was used to store found property and personal belongings.
The Federal Protective Service "also bears some responsibility for the bag ... remaining in the building for 21 days," the report said. An FPS inspector did not identify the explosive during four inspections, noting each time that the post was "clean and orderly" and "free of unauthorized items."
On March 18, 2011 -- 21 days after the bag's discovery -- two guards grew suspicious and screened it. When they could not identify the contents, they notified an FPS inspector, who determined the bag possibly contained an explosive and took appropriate action, the report said.
Following the incident, DECO fired the guard in question.
"The guard's actions were serious breaches that cannot be compared easily to other breaches by this company or other guard service companies because there is little precedent," the report said.
DECO also fired a guard and a supervisor who X-rayed the bag and incorrectly identified its contents. A second supervisor resigned before being fired, the report said. Numerous others were suspended, given written warnings, or retrained, it said.
Both the Federal Protective Service and DECO told federal auditors they have taken steps to avoid a similar lapse. DECO put in place a plan of corrective action and the FPS is extending DECO's contract in three-month increments. It plans to solicit a new contract for guard services in Michigan.