INDIANAPOLIS -- The FBI is stepping in to investigate a physical altercation between two high-ranking Indianapolis police officers and a veteran state trooper, sources told 6News.

Deputy Chief of Investigations William Benjamin and his top aide, Major Chris Boomershine, have given notice of their intent to petition the Fraternal Order of Police union to provide legal counsel.

"Some of the interested parties have confirmed that they were told that there is an FBI investigation," said union President Sgt. Bill Owensby.

Indiana State Police said last month that the department had launched an internal investigation into an "interaction" involving a trooper and an Indianapolis Metropolitan Police Department officer.

Sources close to the investigation told 6News' Jack Rinehart that Master Trooper Detective Wayne Billings -- a 31-year veteran -- went to Benjamin's office at police headquarters last month in plain clothes and identified himself as a state police detective, but never showed a badge or identification.

Eyewitnesses said Billings then made false accusations of a personal nature against Benjamin, and that Benjamin vehemently denied the allegations.

A loud argument ensued, and sources said Billings tried to run out of the office.

An eyewitness said Benjamin called out to Boomershine for help, and Boomershine intercepted Billings, blocking him from leaving the office.

Boomershine asked Billings if he was carrying a weapon, and Billings lifted his shirt, putting his hand on his duty weapon, the source said.

Boomershine then knocked Billings' hand away from the gun and "snatched" the weapon from his holster, the eyewitness said, while other detectives put Billings in handcuffs.

Only after a search of Billings did Benjamin and Boomershine find his badge, identification and a running tape recorder that captured the altercation from start to finish, the eyewitness said.

Sources familiar with the incident said Billings spent an hour in handcuffs in a police interrogation room until his state police supervisors arrived.

Sgt. Anthony Emery, spokesman for Indiana State Police, told Rinehart that the department doesn't believe any criminal behavior took place.

But Owensby said that one of the interested parties told him that he had confirmed through state police and the FBI that the U.S. Justice Department was looking into the matter, and another source confirmed that the FBI has possession of the tape-recorded altercation.

Representatives for Mayor Greg Ballard and Public Safety Director Frank Straub denied asking for FBI help in reviewing the incident.

A representative for the FBI told Rinehart he could "neither confirm nor deny the existence of an investigation."

State police said Billings had been placed on administrative duty as the internal investigation is conducted.