The U.S. Federal Trade Commission's Web site has been down since late morning on Monday, and agency representatives said they don't know what caused the problem.

FTC staffers received an e-mail message Monday saying that the FTC and other U.S. government Web sites were down for an unspecified reason, said an agency spokeswoman. Other major government Web sites, including the .gov sites for the White House and the Departments of State, Justice, Labor, Education and Agriculture, were functioning as of Monday afternoon.

The FTC Web site was down as of about 10 a.m. U.S. Eastern Time Monday and remained off-line at 4:30 p.m.

The office of U.S. White House CIO Vivek Kundra didn't immediately return a phone call seeking comment on how many government Web sites were affected.

The FTC is a consumer protection agency. Among other things, it has filed complaints against companies that have failed to protect their computer networks from intruders.

Another FTC spokeswoman checking on the cause and the length of the outage wasn't immediately able to respond.