( CNN ) -- A U.S. air marshal was assaulted with a syringe containing an `` unknown substance '' at the Lagos , Nigeria , airport on Sunday , according to the FBI , which said it is investigating the incident . The air marshal flew back to the United States after the attack and `` felt fine '' the entire 12-hour flight , a U.S. law enforcement official told CNN . `` Out of an abundance of caution , the Centers for Disease Control and Prevention conducted an on-scene screening of the victim when United Flight 143 landed in Houston early Monday morning , '' a statement from the FBI read . `` The victim did not exhibit any signs of illness during the flight and was transported to a hospital upon landing for further testing , '' the statement said , adding that `` none of the testing conducted has indicated a danger to other passengers . '' Investigators are looking at whether the air marshal , who did not exchange words with the attacker before the incident , was targeted because he 's American . However , there is no concern at this point in the investigation that the attack is part of a bigger threat , according to the source .
The air marshal was injected with a syringe containing an ` ` unknown . '' He was transported to a hospital upon arrival in the United States .