A critical germ lab at the Centers for Disease Control and Prevention lost power last week because the agency had taken two backup generators out of service for upgrades, CDC officials said Friday.

Their absence from a complex, centralized backup generator system created a power fluctuation when the system was activated during a July 11 power outage, causing the whole system to shut down, CDC spokesman Dave Daigle said. The problem is being fixed, he said.

The backup power failure — the second in 13 months — is the type predicted years ago by some CDC engineers. And it has heightened concerns in Congress about lab safety at the Atlanta agency, which experiments on smallpox, Ebola, anthrax and other deadly germs.

"For high containment labs, repeated power failures are repeated safety failures," said U.S. Rep. John Dingell, chairman of the House Committee on Energy and Commerce, which has been investigating biolab safety nationally.

"Fortunately, there were no adverse consequences this time," Dingell (D-Mich.) said Friday. "However, the fact that these incidents continue to occur raises serious concerns about the future and highlights the fact that CDC must act now to address problems in its system."

Last week's incident began when a bird shorted out a Georgia Power transformer about 5:40 p.m., cutting off power to part of the CDC's main campus on Clifton Road. The agency's backup generation system initially came on, but quickly shut down, and power remained off for one hour and 15 minutes at four agency buildings.

Three were office buildings. But the fourth, Building 17, housed infectious disease labs, where scientists work with the H5N1 avian flu virus and other dangerous germs. Without power, the labs can't run negative airflow systems that help contain germs in Biosafety Level 3 labs, such as those in Building 17.

CDC officials have said neither workers nor the public was at risk, and that the labs have many other safety systems that don't require electricity. At the time of the outage, no work was going on with high-risk pathogens such as flu, tuberculosis, meningitis or rabies, Daigle said.

The backup generator system is being recalibrated to work with the two missing generators, Daigle said. A test is scheduled for 5 a.m. Monday. After the two generators were removed in April for "refurbishment," tests showed the system worked, Daigle said.

In June 2007, another new CDC lab tower, Building 18, lost power for about an hour after a lightning strike.

CDC construction officials had warned since 2001 against removing generators from critical lab buildings and clustering them to create a centralized system, The Atlanta Journal-Constitution reported last summer. But agency officials have said the consensus among other experts supported the centralized system.

In a September 2002 e-mail, CDC mechanical engineer Johnnie West wrote: "I have very little confidence in the generators being able to operate as designed, due to the complexity of so many generators being connected."

In an August 2003 e-mail, West warned that linking the generators in a centralized yard created a "slim" likelihood they would work because "they are all controlled by one control system. This system hasn't worked properly since it was installed 4 years ago," he wrote.

"Even though we espouse that the laboratories are the most critical, anyone looking at the electrical system would question that."

West, who has previously declined to be interviewed, could not be reached Friday.

Daigle said that the CDC has completed a recent assessment of the reliability of its centralized backup generator system. He said the agency would soon release that information to congressional investigators.

The Government Accountability Office, the investigative arm of Congress, has been examining the concerns raised by West and others as part of its probe of the safety of U.S. labs working with potential bioterrorism agents.

To reach staff writer Alison Young, call 404-526-7372.