Signs warn against trespassing onto the Y-12 National Security Complex in Oak Ridge, Tenn., on Aug. 17, 2012. (Erik Schelzig/AP)

Nearly two years before peace activists broke into a U.S. nuclear weapons facility in late July, government investigators warned in classified reports of lax security at the complex where the nation’s largest concentration of weapons-grade uranium is stored.

The previously undisclosed 2010 criticism of the Y-12 National Security Complex near Oak Ridge, Tenn., found that security cameras were inoperable, equipment maintenance was sloppy and guards were poorly trained.

On July 28, an 82-year-old nun, an elderly gardener and a house painter slipped past security costing millions of dollars a year — sensors, cameras, barbed wire, heavy weaponry, roving patrols and dozens of guards — to reach the walls of a new uranium storage facility at Y-12. They splattered blood on the building and held up protest signs before being arrested.

Although the three intruders never got close to the nuclear material, which is kept behind 30-foot concrete walls and hidden gun ports, the break-in was a huge embarrassment for the National Nuclear Security Administration, the Department of Energy agency that manages the nuclear weapons stockpile and spends $1 billion a year securing it.

It was also an indication that the DOE may have had weak follow-up of problems identified almost two years earlier by its own investigators.

Classified reports dated Oct. 1 and Nov. 19, 2010, prepared by the department’s independent safety and security investigators, highlighted security lapses similar to those blamed for the July break-in, according to officials who have seen the reports and spoke on condition of anonymity because the documents are classified.

The House Committee on Energy and Commerce’s oversight subcommittee is scheduled to hear testimony from DOE officials at a public hearing on Wednesday morning.

A report by the department’s inspector general issued last month blamed the break-in on “multiple system failures on several levels” and criticized both the DOE and the outside contractors who operate security.

Among the most intriguing lapses identified by investigators is that the 700 sensors surrounding the high-security zone routinely register 200 false alarms a day, usually from squirrels, deer and other animals in the wooded valley between two ridges surrounding the complex, according to one government official. The false alarms numbed guards to the first line of defense, investigators found.

After the incident, the complex installed fencing to keep out animals and the number of false alarms immediately dropped by 50 percent, according to one government official.

Like the nation’s other nuclear weapons facilities, Y-12 has been run by private contractors since 2008. Its main operator, Babcock & Wilcox, subcontracts with Wackenhut Services for its protective force.

Managers and security personnel have been removed or reassigned following the break-in and at least four federal agencies are continuing to investigate security at the sensitive complex.

Joshua McConaha, spokesman for the NNSA, said DOE Secretary Steven Chu “has made clear, the recent incident at Y-12 was a completely unacceptable breach of security, and an important wake up call for our entire complex. The severity of the failure of leadership at Y-12 demands swift, strong and decisive action by the Department.”