A former Transportation Department GS-5 employee who stole $850,000 from the agency this summer and then spent it in a two-month spree, pleaded guilty yesterday to two criminal counts.

William C. Sibert could be sentenced to a maximum of 15 years in prison and fined up to $2,000 for the offenses. He is already serving a six-month jail sentence on an unrelated gun possession charge imposed last week in Virginia.

Sibert, 29, entered his plea in federal court here after Assistant U.S. Attorney Robert W. Ogren of the fraud division described his theft as a crime that could have been prevented by tighter controls within the Transportation Department.

Ogren said in court Siberthad bragged to coworkers in his branch at the Urban Mass Transit Administration that his supervisor would sign anydocument put before him without reviewing it.

Sibert wasin a position to generate grants for his supervisor to approve to various mass transit organization, and received more than $850,000 in Treasury Department checks to his name by including himself among the persons to receive such grants.

Sibert said himself in court yesterday that his name was included on the grant forms when they were signed by his supervisor, identified in court as Lewis Keeton.

When U.S. District Judge Howard F. Corcoran appeared amazed that such a thing could occur, Sibert's attorney, Ed Wilhite, said, "I believe Mr. Keeton, the approving officer, didn't read it (the form), your honor."

The government said in court it had no proof that Sibert's name was on the forms when Keeton approved them, and said the name could have been added by Sibert after Keeton's approval.

Keeton, reached by telephone at UMTA yesterday, also discounted Sibert's claim that his name was on the form when he signed it. "There's proof to differ with that. Ican't say anything more about it."

Transportation officials and other investigators said Keeton had been transferred since Sibert's theft into a position outside the grant certification process. Keeton said, however, that he was in "very much the same job. There's some change, but very little."

Although transportation officials said they have not yet received a full report from investigators who worked on the Sibert case, they said they had changed some procedures in an attempt to make any similar theftsmore easily caught in the future.

One investigator who worked on the Sibert case and has handled other government theft cases called it "a typical Washington story."

"Single agencies generate incredible sums of money, and don't build in the checks and balances to detect theft," said the investigator. Sibert's agency alone handled $2 billion a year in grants, the investigator added.

Another federal investigator speculated that Sibert would have stolen even more money from his agency without fear of apprehension if he had been more cautious in how he spent it.

Sibert, was arrested in late July after his bank, Suburban Trust Co., notified the Treasury Department that he was receiving huge sums of government money to which they did not know if he was entitled.

At the same time, internal Transportation Department investigators said they were begining an investigation of Sibert after he began showing up daily in new suits, bragging about his new luxury cars and taking large groups of coworkers out to expensive lunches.

Further investigation after Sibert's arrest showed he had used the stolen money to buy 16 luxury cars, a new house and furnishings in Maryland, numerous expensive household items and stocks, and a topless bar about a block from the FBI headquarters here. He and his wife lived in Clinton, Md.

The government has placed civil attachments on Sibert's property pending the sorting out of the purchases and property bought with government funds will be auctioned.

Sibert's guilty plea was to one count of mail fraud and one count of making false statements on government documents.