A U.S. federal air marshal has been quarantine in Houston after being attacked Sunday by an unknown assailant wielding a syringe at an unsecured area of the Lagos, Nigeria airport, ABCNews reports.

The assailant injected an unknown substance into the arm of an air marshal who was traveling with a team of other marshals, U.S. law enforcement officials told ABC News.

The marshal traveled back to the U.S. after the attack on the United Airlines flight on which he was scheduled to work and was met early Monday morning in Houston by FBI agents and health workers from the Centers for Disease Control.

The air marshal was immediately put into quarantine over fears the substance could contain some form of the Ebola virus. The FBI said he was screened “on-scene…
out of an abundance of caution,” ABCNews reports.

“The victim did not exhibit any signs of illness during the flight and was transported to a hospital upon landing for further testing,” an FBI spokesperson said. “None of the testing conducted has indicated a danger to other passengers.”

Most infectious agents won’t manifest or make the patient contagious immeditaely, health experts said, according to ABCNews.

The unknown assailant got away, but officials said other air marshals brought the needle back to the U.S. for testing.

U.S. air marshals travel undercover in plain clothes and it would take an insider to alert an attacker that his target was an American law enforcement agent, officials said.

“While there is no immediate intelligence to confirm this was a targeted attack, this is our reminder that international cowards will attempt to take sneaky lethal shots at our honorable men and women abroad,” said Jon Adler, the national president of the Federal Law Enforcement Officers Association.

The Lagos airport has long been considered a possible target for the terror group Boko Haram which has conducted a series of deadly attacks across Nigeria, ABCNews reports.