The Centers for Disease Control and Prevention simulated a blackout on its Clifton Road campus early Monday to test its backup power generation system, which has failed twice in the past 13 months.

CDC officials would not release details of the test results. In general, Monday's tests identified "a few more tweaks we need to make and we're making them," said agency spokesman Tom Skinner.

The tests occurred from 4 a.m. until 5:30 a.m. and all power was restored at the end of testing, he said.

Skinner would not say whether backup power worked properly for key infectious-disease labs during Monday's tests.

On July 11, CDC lab Building 17 was without electricity for one hour and 15 minutes after backup generators shut down during a real power outage.

In June 2007, a lightning strike caused an hourlong power outage at another lab tower, Building 18.