The FBI is investigating whether about 15 of its field office chiefs improperly billed the government for expenses in traveling to the retirement party of ousted Deputy Director Larry Potts, Justice Department and FBI officials said Friday.

At issue is whether these special agents in charge, known as SACs, did government business when they traveled here from their field offices for the Oct. 9 private retirement party for Potts at a hotel in the northern Virginia suburbs, according to several officials.

In consultation with its Justice Department counterpart, the FBI’s Office of Professional Responsibility, an internal watchdog unit, “is investigating allegations a few of its employees were involved in improper billing of the government for travel costs to attend a retirement dinner,” Justice spokesman Bert Brandenburg said.

Potts, a popular figure inside the FBI, spent the last two years of his career on paid suspension. He retired last August shortly after the Justice Department decided not to prosecute him for his role in supervising the 1992 siege on Ruby Ridge in Idaho. During that episode, an FBI sniper killed the unarmed wife of white separatist Randy Weaver. When Potts left, Justice officials were still deciding whether to fire him over that role.