When Richard Rios showed up Monday for his last full day as head of the Community Services Administration, he found the door to his downtown Washington office bolted from the inside.

When Rios and aides finally got intothe office, they discovered that a stack of documents involving last-minute agency business was missing from his desk.

Among the papers taken from the administrator's office were files related to CSA grants to the United Farm Workers union and a federal suit, brought by the American Farm Bureau Federation, attempting to thwart them.

Rios, contacted in California, said yesterday that some of the missing papers were to have been used by him later Monday in giving a deposition in the Farm Bureau suit. The deposition session was postponed.

Rios said he had no theories on who might have been in his office or what they might have been seeking. Detectives from the General Services Administration's Federal Protective Service were interviewing CSA employes yesterday.

"There were some plans for funding of grants in the pile of missing papers. People might have been very interested in any grants we might be giving on the last day," he said.

Among his last-minute decisions was one to release $125,000 to the UFW for establishing a credit union. Release of the money was halted later Monday when the Farm Bureau obtained a temporary restraining order in federal court. Rios said he was unaware of any connection between the loss of the credit union data and the restraining order, issued subsequently.

The Farm Bureau also is objecting to an earlier CSA grant to Cesar Chavez's UFW to set up a microwave communications system. Questions have been raised about UFW's management of the grant.

Rios said he felt the intruders had taken no irreplacable documents. "Whoever did it didn't do a very good job," he said. "They should have broken in about two weeks ago -- I had a lot more papers then."

The thieves apparently gained entrance to Rios' executive office through a side door, and bolted the main door from the inside before departing.

Rios and other CSA officials said that the security register in the lobby of the building at the corner of 19th and M Streets NW indicated that two persons had signed in at 5:30 p.m. Sunday "to visit CSA offices" on the floors above. The log showed they left the building at 6 p.m. Rios and others said they did not recognize either of the listed names.

A spokesman for the Federal Bureau of Investigation said yesterday that its Washington office had not been notified of the incident at CSA.