The planned opening of the Capitol visitor center, already more than a year behind schedule, might be pushed back again due to holdups on major safety systems and other unknowns, according to a government watchdog agency.

Construction on the 580,000-square-foot addition has accelerated, but "the proposed opening dates do not allow enough time to complete several critical activities and to address problems, challenges, risks, and uncertainties," the Government Accountability Office reported Friday.

The agency said it would reassess the project's schedule this summer. Plans now call for the center to open in April 2007. It was originally supposed to be completed in time for the presidential inauguration in January 2004.

The Capitol visitor center has come under criticism from lawmakers and taxpayer watchdog groups because of delays and mounting costs. When first designed in 1995, the center was projected to cost $95 million. The cost has now climbed to more than $500 million.

The center was originally intended to improve visitor convenience and building security. But the Sept. 11, 2001, terror attacks led to an expansion of the project, which now includes a 450-seat auditorium that could serve as an emergency legislative chamber.

The GAO, which has been providing regular project updates to the Senate Appropriations subcommittee on the legislative branch, warned of potential complications with the center's complex heating, air conditioning, ventilation, fire protection and security systems.

If the construction team "encounters any significant problems with their functioning, either individually or together ... the project could be seriously delayed," the GAO said.

Still, despite some "schedule slippage," Architect of the Capitol Alan Hantman said there was no change to next spring's estimated opening date.

"While the pace of construction sometimes doesn't meet our expectations, our contractors do continue to make steady progress throughout the facility," Hantman said in his testimony.