A fired employee armed with a shotgun held four people hostage in a Postal Service office Wednesday morning and forced the evacuation of part of a major regional mail center, police said.

The gunman was fired for threatening a supervisor, said John Freeman, a postal inspector.Police negotiators began talking with the man by phone, and he seemed interested in maintaining contact, although he abruptly cut them off at one point, police Sgt. Dennis Cribari said.

No shots were fired, said Steve Green, manager of marketing for the Postal Service's Denver District.

Freeman said the gunman entered the building through a loading dock area.

Cribari said three women and a man were taken hostage in a supervisor's office at the facility that handles most mail processing for Wyoming and Colorado. About 3,000 people are employed at the center, which includes a 24-hour post office.

Not all those on duty were evacuated because police were able to isolate the area where the hostages were held, Cribari said.

Jeff Pecosky, who works at the complex, said he was leaving on a shift change when people began rushing from the building. He said the situation unfolded in an area with security restrictions, which employees need a badge to enter.

The entire complex just northwest of the old Stapleton Airport is about the size of eight football fields. It has a day care center on the opposite side of the building, and that was safely evacuated.

Al DeSarro, spokesman for the Postal Service's Western Region, said mail had already been moved to post offices, so there should be no interruption of service during the busy holiday season.

Postal employees have been involved in several deadly encounters in recent years.

The worst was in August 1986 when Patrick Henry Sherrill, a part-time letter carrier in Edmond, Okla., killed 14 colleagues and himself.