Maryland State Police arrested Thursday night a man who allegedly used at least 15 aliases during recent years to slip in and out of a variety of jobs - the most recent being a physician at the Maryland State Penitentiary Diagnostic Center.

Police officials said yesterday the man's background remains almost a total mystery, and that they had not determined his identity. The one fact that they said they had to work with was that their suspect was middle-aged.

"We know this person is the same person posing as the physician at the pen," State police spokesman Bill Clark said. "His true identity? We have no idea."

Clark said for the moment officials are referring to the man as Leon Collier, age 41, because they were able to track him down by tracing mail addressed to him by that name. However, Clark said Collier identified himself as Robert E. Lee Pugh Jr. when he was arrested.

When State Police arrested Collier-Pugh in a car in the 2400 block of Jameson Street in Temple Hills, they also confiscated "a mound of material" that included different medical instruments, several different identification papers, Social Security cards, mail order college degrees, and prescription drugs, Clark said.

Yesterday, investigators from the Federal Drug Enforcement Agency, the Internal Revenue Service, the District of Columbia police fraud squad and the Baltimore City State's Attorney's Office sifted through the material "in an attempt to unrvael his mysterious background and activities," Clark said.

Clark said Collier-Pugh posed as a doctor when he applied successfully for the prison post. State Police were asked by prison authorities to investigate Collier-Pugh after becoming suspicious of irregularities in his personnel file.

Collier-Pugh was then indicted by a Baltimore grand jury for impersonating a physician and on 34 counts of dispensing a drug without a license. State Police officials went to an address supposedly belonging to Collier and found a note saying that because of mail theft his mail was to be delivered to the Anacostia Post Office in Southeast Washington.

Clark said Collier-Pugh was receiving workmen's compensation checks.

That information sent police officials to the D.C. Department of Human Resources, where they learned Collier-Pugh was receiving checks for an injury he received while working at Cedar Knolls, the city's children's center. Clark said Collier-Pugh was arrested near the address he gave DHR officials.

Clark said that Collier-Pugh apparently had worked at various agencies in Baltimore and Washington, claiming at various times he held a Ph.D. Clark said Collier-Pugh left the prison job when authorities learned that his birthdate wasn't the same as that of the doctor he was allegedly impersonating. Collier-Pugh blamed it on a bureaucratic mixup, promised to remedy it, and promptly disappeared.

He was arrested in a car belonging to a Washington, D.C., woman, State Police officials said, but they had no other information on who the woman was.

Using a search warrant, State Police officials searched another car nearby which they said Collier-Pugh used to drive to work at the state prison and found the voluminous materials, which allegedly included many federal, Maryland, and Virginia drug dispensing licenses and prescription blanks. Drug Enforcement officials said Collier-Pugh also had copies of the federal and state registration belonging to a Baltimore doctor. Such registration designates what kinds of drugs a doctor may prescribe.

State police officials said Collier-Pugh is being held in the Baltimore City jail.