FMCSA said in an e-mail that FBI sources indicated the action was precautionary, and not in response to any direct threat.
According to a report on the Las Vegas Review Journal website, Hoover Dam is open to passenger cars only. The visitors center and parking areas remain closed, the website said.
U.S. Route 93 runs over the Hoover Dam, which is on the Nevada-Arizona border. It is 30 miles Southeast of Las Vegas.