Updated 6:20 p.m.

The FBI is investigating a suspicious package containing explosives left unattended by security guards for three weeks inside a 26-story federal building in Detroit, according to law enforcement officials.

The package, containing “explosive components,” is at the FBI crime lab in Quantico for further testing as a federal investigation continues, according to FBI Special Agent Sandra Berchtol.

The Patrick V. McNamara Federal Building in downtown Detroit houses regional offices for the FBI, IRS, Social Security Administration, Peace Corps and Sen. Carl Levin (D-Mich.). Law enforcement sources blamed the incident on private security guards and inspectors with the Federal Protective Service, a tiny Homeland Security agency responsible for protecting more than 9,000 federal facilities nationwide.

A law enforcement source said the suspicious package was brought by a private security guard into the building, where it remained, unopened and unscreened, for about three weeks. The source, who spoke on condition of anonymity because the investigation is ongoing, said security officers thought the package might have belonged to someone on a nearby construction crew.

About three weeks later, security personnel decided to screen the package, became alarmed at the results and called federal authorities and the Detroit police department’s bomb squad. “They weren’t sure what it was, but thought it better to get it out of the building,’’ the source said. “That’s when everyone converged on the building.”

Asked why the package was not screened for three weeks, the law enforcement official said: “You’d think they would have screened it.’’

A private security guard involved in the incident has been suspended until further notice, according to a senior FPS official. Similar disciplinary action will be taken against contract and FPS-employed guards if the investigation determines others are culpable, the official said.

Levin and his staff have been briefed on the ongoing investigation and he "is eager to learn the results of that investigation," a spokeswoman said.

The Department of Homeland Security did not immediately respond to requests for information.

FPS employs about 800 federal inspectors who are responsible for overseeing more than 15,000 private security guards and for drafting building security plans. Friday's incident is the latest in a string of embarrassments for the agency.

The Government Accountability Office reported in 2009 that it successfully smuggled bomb-making materials into 10 federal buildings protected by FPS that house national security or law enforcement responsibilities. GAO is conducting multiple ongoing investigations into the agency's activities and is exploring potential alternatives to FPS, according to a GAO official.

The agency — once overseen by Immigration and Customs Enforcement and now managed by DHS’s risk-reduction office — is deploying certified trainers to provide refresher courses to contract and FPS guards across the country, according to the FPS official, who was not authorized to speak publicly. The training team is scheduled to meet with Detroit-area guards in the coming days.

Democratic lawmakers introduced legislation last fall that would require FPS to establish national training standards, hire more federal inspectors and explore ways to federalize contract guards. The legislation never made it out of House and Senate authorizing committees.

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This post has been updated since it was first published.