A U.S. Customs Service official said yesterday that his agency is investigating allegations of currency violations before a federal grand jury that has focused its attention on Alexandria Commonwealth's Attorney William L. Cowhig.

William P. Rosenblatt, acting director of currency investigations for the service, said that the agency is attempting to determine if more than $5,000 in U.S. currency was ever taken out of or brought into the country without being declared on forms that international travelers are required to sign.

Rosenblatt declined to discuss specifics of the investigation, but referred questions to a federal prosecutor in Alexandria who Monday began a grand jury investigation into allegations of political corruption connected to Northern Virginia's once-flourishing bingo and massage parlor opetations.

Cowhig, the state prosecutor in Alexandria since 1972, said last night he knew nothing of the customs service investigation. He said he has never taken as much as $5,000 with him out of the country.

In April Cowhig requested the appointment of a special prosecutor to investigate alleged violations of bingo and gambling laws amid criticism that he had been lax in prosecuting violators under Virginia and local laws. He has denied any wrongdoings and said he views the current federal investigation as a "political hatchet job."

The customs investigation became public Monday when a witnessbefore the federal grand jury said later that a customs official had shown him photographs taken in the Bahamas of several Alexandria residents who were staying at a hotel owned in part by cowhig.

The witness, William H. Fields, a businessman who is the chief aide to Alexandria City Council member Nicholas A. Colasanto, said he had recognized the locations in the photographs as being that of the Two Turtles Club, which Cowhig and several relatives have owned for at least a decade.

Separately it was learned that after having been elected to office, Cowhig invited several dozen Northern Virginia lawyers and others to a meeting at a waterfront club in Alexandria at which he tried to persuade them to purchase a time-sharing arrangement for use of facilities at the Two Turtles Club.

A former Virginia prosecutor asked yesterday about the meeting, said he believed that a prosecutor would place himself in a possible conflict of interest by seeking monies from lawyers whom he might have to oppose in court.

According to one person who was present at the meeting, Cowhig flashed colored slides on a screen before the group, which also included several physicians. The slides showed the main building of the flagstone compound, as well as three detached cottages, all of which sits on property that fronts on a lagoon.

The man recalled, "Bill (Cowhig) said, 'This is what we've got, down there,' and asked us if anyone was interested in a time-sharing arrangement whereby you rent the cottages for a specified amount of time.

". . . 'Then we'd be happy to have you come down and look at it," Cowhig told the group, the man said.

Cowhig said his recollection of the meeting is hazy. "Maybe we had a small group together to discuss it. We wanted to syndicate it with the lawyers in Alexandria, but unfortunately no one wanted to invest in it."

According to Fields, one of the people who visited the Two Turtles Club on the island of Exuma was James R. Fike, who was recently indicted on felony gambling charges linked to bingo games. Fields said he identified Fike from a photograph shown to him by customs officials.

Cowhig has acknowledged that in 1972, he acted as the incorporating attorney for B & J Specialties, a company that Fike was establishing. Fike has been accused of conducting illegal gambling in the name of B & J Specialties from March 1, 1977, through Jan. 17. He has denied the allegations.

The Two Turtles Club was built in the late 1960s by a Bahamian businessman and purchased by Cowhig and others shortly after it was finished, according to Revis Turnquest, who manages the club on a lease arrangement with Cowhig. The original purchase price could not be learned yesterday but Cowhig said the club is now worth about $250,000.

The club has "never been too successful, although we are hopeful we can change things," said Turnquest's wife.

The three cottages on the property rent for as much as $49 a night during the tourist season, which coincides with the North American winter, according to Audrey Clark, a hotel employe.

The club is about four miles from the Georgetown airport, which can accommodate both private and commercial airplanes, Clark said.

The grand jury yesterday heard testimony from the Rev. John Adams, a Catholic priest, who said earlier that late last year he received a telephone call from Alexandria City Council member Nicholas A. Colasanto. suggesting bingo as a fund-raising device for a charity group with which Adams was then involved.

Adams said he later received a call from someone whose name he does not recall, concerning bingo. "When I asked him to put in writing the terms of the arrangement, what the overhead would be, what our profit would be, the man refused," Adams said he testified to the grand jury.

Also testifying was George Babar, president of Alexandria Volunteer Fire Company No. 5.

Babar said before entering the grand jury room that bingo operators had guaranteed his group $50 each night, but said he later learned they grossed as much as $3,000 nightly.