A 13-year-old Columbia youth's voyage into the world of computer piracy has been an eye-opening experience for local police, who say their computer expertise pales in comparison to the self-taught skills of the byte-size buccaneer.

"I never even touched one of these until we got this one," said Howard County police Lt. Michael A. Chiuchiolo, holding the keyboard of the youth's confiscated home computer.

County police, acting on a tip from an informant, arrested the unidentified youth last month for allegedly using data supplied by computer pirates from around the country to make illegal credit card purchases, place free long-distance telephone calls and obtain hundreds of bootlegged computer programs worth as much as $20,000.

The case has called into question the ability of local police departments to investigate computer banditry, which some experts consider the fastest-growing crime of the 1980s. In Howard County, home to thousands of white-collar professionals who work in high-tech industries, the police were caught cold.

After obtaining a warrant, the officers searched the youth's Columbia home and seized his computer equipment, merchandise allegedly purchased with the credit card numbers and at least 80 computer discs on which data was stored. But none of the officers knew how to operate the Commodore 64, a model that sells for about $179, even though it is considered an easy-to-use model, Chiuchiolo said.

County police had to call in an expert from the FBI to give them a crash course in computer programming. The youth, who authorities said was cooperating with police, has given several demonstrations, Chiuchiolo said.

Local police are responsible for investigating such cases. The FBI provides only technical assistance, an FBI spokesman said.

The Secret Service is involved in this case, but only to investigate alleged credit card fraud, said special agent James E. Le Gette.

Recently, in a conference room at police headquarters, Chiuchiolo demonstrated the youth's equipment, gingerly hunting and pecking on the computer keyboard to call up programs.

He said it may be weeks before all of the information stored on the computer discs is examined.

"A lot of them are programs made up by other computer hackers. He even designed a computer game himself which is really quite good," the officer said.

The youth, who is described by police as an otherwise "typical" suburban teen-ager, got the home computer last year and read dozens of books and magazines on the subject.

Five months ago, he added a disc drive unit, which can retrieve information stored on small discs inserted into the machine.

After that, police said, he embarked into the world of computer piracy, apparently linking with 15 to 20 computer hackers around the country. The hackers, known by code names such as "The Kernel" and "The Bandit," exchanged information and programs through so-called computer "bulletin boards."

Stored on the youth's collection of discs were scores of allegedly pirated games such as the familiar "Donkey Kong" and "Space Invaders," police said. They also found illegal programs that can place free long-distance calls on AT&T's own corporate telephone lines, simulate the tones of coin-operated phones or get access to Sprint, MCI and other independent phone companies, Chuichiolo said.

Telephone company investigators watched in amazement as the youth effortlessly placed a call to London during one of his recent demonstrations, the officer said.

The teen-ager also had a program to conduct "dirty dialing," a term used for a computer that dials every telephone number in a given exchange in search of other computers with which his could be connected, police said.

Dirty dialing was highlighted in the movie "War Games," in which a teen-age computer wizard inadvertently gains access to Defense Department computers and almost starts World War III.

The officer noted that one of the Columbia youth's programs is titled "NORAD."

"We hope it's just a game," he said.