Investigators declined to release further details about the incident, which occurred in late February, saying the investigation is ongoing.
City spokeswoman Rebecca Rodriguez said she is unable to share details because they may compromise the police investigation.
"It was at the request of law enforcement that the city of Arlington did not disclose to the public that this incident had happened," Rodriguez told The Fort Worth Star-Telegram. "The public has not lost a penny."
The Star-Telegram said the investigation has been handed over to the FBI, and the city is reviewing security measures to safeguard against another incident.
"Given the times in which we live, it is wise for the city of Arlington to constantly reassess the threats which may exist or may be just over the horizon," Rodriguez said.