A U.S. air marshal was undergoing tests in a hospital in Houston after being assaulted in Lagos, Nigeria with a syringe containing an unknown substance, the Federal Bureau of Investigation said on Monday.

The marshal was attacked on Sunday at the airport in Lagos and given an on-scene screening by the Centers for Disease Control and Prevention before boarding a United Airlines flight to Houston that landed on Monday, it said.

"The victim did not exhibit any signs of illness during the flight and was transported to a hospital upon landing for further testing. None of the testing conducted has indicated a danger to other passengers," it said, without providing further information.