A U.S. grand jury in Alexandria yesterday began an investigation into bingo operations, sexual massage parlors, and possible political corruption in Alexandria by focusing its attention on Alexandria Commonwealth's Attorney William L. Cowhig.

The first witness before the grand jury, Alexandria businessman William H. Fields, said he was asked repeatedly by federal prosecutors about his knowledge of airplane trips Cowhig has taken over the years to a club Cowhig owns in the Bahamas, and about the possible "laundering of money" there.

Fields said he also was asked about airplane trips made to the Bahamas by other leading Alexandria political and legal figures, including former Virginia delegate James M. Thomson. Fields said he had no direct personal knowledge of the trips, although he said he had been told about them several weeks ago by U.S. Customs agents.

Cowhig, reached at his home yesterday, acknowledged that he and several relatives have owned The Two Turtles Club on the Bahama island of Exuma, in the town of Georgetown, for about 10 years. He said he and other northern Virginia politicians and lawyers have traveled there frequently in the past.

But Cowhig categorically denied that any of these trips involved anything illegal. He said he knew nothing about the "laundering of money" that Fields was questioned about.

Cowhig described the Two Turtles Club as "more like a rooming house . . . just a vacation place." He said it has three rooms, a bar, a restaurant and a game room. There is no gambling at The Two Turtles Club, Cowhig said, although there are gambling casinos elsewhere in the Bahamas.

"Anyone who would try to insinuate anything wrong (about his part ownership of the club) is a damn liar," Cowhig said.

Cowhig said he has never been questioned by state or federal prosecutors about either bingo or massage parlor operations in Alexandria.

Cowhig is now in his second term as Alexandria's chief prosecutor. He is the stepson of the late James Colasanto, a judge in General District Court in Alexandria's Two Turtles Club with two brothers and a sister, he said yesterday.

Fields, 51, is the chief political aide to Alexandria City Councilman Nicholas A. Colasanto, the brother of the late judge. Fields also is a businessman and accountant who until 1975 prepared and signed Cowhig's tax returns, he said he told federal prosecutors.

In mid-1977 Cowhig had helped organize an Explorer Scout troop which intended to raise funds for flying lessons by staging bingo games, he has said previously.

When Explorer Scout officials protested the involvement of the group with bingo, the bingo games were stopped, Cowhig and Fields have said previously.

Fields staged several bingo games at a bingo parlor on Mt. Vernon Avenue in Alexandria, but then withdrew from participation, he said.

After nearly six hours of testimony yesterday before the newly-empaneled grand jury, Fields said federal prosecutors asked him about his personal knowledge of trips Cowhig, a licensed pilot, made to the Bahamas in a private plane, and whether Cowhig was accompanied by James I. Burkhardt, an Alexandria attorney who has represented leading figures in Northern Virginia's bingo and massage parlor operations.

Prosecutors also asked him, he said, if he knew of nay trips Cowhig had taken to the Bahamas with Thompson, for years one of the most powerful legislators in Richmond.

Fields told prosecutors he had no personal knowledge of Burkhardt or Thomson being in the Bahamas, he said. Outside the grand jury room Fields said that U.S. Customs agents several weeks ago had shown him pictures of Burkhardt, Thomson and other at a location "I know to be the Two Turtles Club."

Cowhig and Thomson said yesterday that both had flown to the club by commercial airliner after a recent session of the Virginia General Assembly.

Burkhardt could not be reached for comment, and prosecutors declined to comment.

Federal prosecutors have subpoened at least half a dozen people who were involved with the Explorer Scout troop, or with other bingo games conducted in the Alexandria area during the past several years. Because of Field's lengthy testimony in the grand jury room, these people are expected to return today to testify.

Cowhig requested the apointment of a special bingo prosecutor several months ago after some members of the Alexandria City County charged he was moving too slowly to prosecute violations of the city's bingo laws. Since his appointment, special prosecutor Edward L. White has obtained several misdemeanor convictions and theindictments of two persons on felony gambliing charges.