An on-duty Federal Air Marshal was stabbed with a syringe containing an unknown substance at the airport in Lagos, Nigeria, on Sunday. The marshal then boarded a flight and flew to Houston, the FBI and TSA announced Monday.

The marshal was not severely injured by the needle prick, but it was unclear if it contained any hazardous or viral substances. The FBI said that 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."

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The marshal, whose name has not been revealed, was then taken to a hospital for further testing, the FBI said. The marshal did not exhibit any signs of illness during the flight and the subsequent tests — including for HIV and Ebola — did not indicate the marshal was a danger to other passengers, officials said.

It appeared the stabber did not know the victim was an air marshal and it may have been a robbery attempt, sources told NBC News.

Sources told NBC Houston affiliate KPRC that "a group of federal agents in plain clothes were about to walk through security at the Lagos airport Sunday when they were approached by a group of men. Sources say one of them stabbed a needle into an agent's arm and then they all ran away." The agent then pulled the needle out, and then stored it in a safe container, the sources said. The FBI is now testing the needle.

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— Tom Costello, Jonathan Dienst and Hasani Gittens