A U.S. District Court jury began deliberations here yesterday in a major drug distribution case in which former Washington lobbyist Fred B. Black Jr. is accused of laundering $1.5 million in drug profits through Riggs National Bank for a Bethesda cocaine kingpin.

Prosecution testimony in the trial, now in its eighth week, has included stories of Miami-area drug smuggling, cocaine deliveries by private plane, suitcases stuffed with hundreds of thousands of dollars and even the acknowledged Bethesda dealer pulling a wad of money out of his boot for a bank deposit.

The case also is tied to a recently extradited Colombian citizen, Marcos Cadavid, who is scheduled for trial next week and is under extraordinarily heavy guard by U.S. law enforcement officials who fear he may be the target of assassins dispatched to this country by angry Colombian cocaine czars.

A federal grand jury in Washington indicted 15 people in December 1983 in connection with the alleged conspiracy to distribute cocaine here and in several other states.

Seven, including the government's star witness in the current trial, have pleaded guilty. Two others are fugitives. Four defendants are now on trial and Cadavid and former Riggs vice president William G. Hessler face trial later this month.

Federal prosecutors have alleged that Black, who lives at the Watergate apartment complex, conspired to launder more than $1 million in cocaine proceeds through two dummy accounts at a Riggs branch at the Watergate, which Hessler managed.

Much of the money was later invested in a proposed 13-story, 515-room Atlantic City casino, according to prosecutors.

Prosecutors charged that Black received the cash from Bethesda drug dealer Lawrence G. "Lonnie" Strickland, whose sister is married to Hessler. Strickland, who pleaded guilty earlier, has been the government's key witness.

Also on trial are Robert H. Burns, a Miami lawyer charged with selling cocaine wholesale to Strickland; John C. Tarantino, assistant city attorney of East Orange, N.J.; and Wilfred Samuel Bell, described by prosecutors as "the biggest outlet for Strickland's cocaine in the Washington area."

Bell, who left the United States for England before the 1983 indictment, was extradited to stand trial here. All four men have pleaded innocent.

Black's lawyer, Thomas Dyson, contended in closing arguments that Black never profited from cocaine proceeds. Instead, said Dyson, Black was "struggling" financially.

Black's "biggest struggle," scoffed Assistant U.S. Attorney Roger Adelman, was carrying the suitcases of cash brought to him for laundering by Strickland.

Adelman and Assistant U.S. Attorney Paul Knight charged that Black made several trips to Riggs branches in one day trying to break Strickland's cash bonanza into smaller, less noticeable deposits.

Strickland testified he met Black through the Hesslers in the mid-1970s when Black needed a loan. Strickland told his sister to retrieve $17,000 from a shoe box at "Nanna's" -- the home of Strickland's grandmother -- for Black's use.

The loan later became "an investment" in the proposed casino as the business relationship between Black and Strickland blossomed, Strickland testified.

In September 1979, as cash demands for the casino project intensified, Strickland dispatched $304,000 in cash for laundering by Black, according to testimony.

A second cash shipment of $500,000 from Strickland to Black was flown by a private pilot into National Airport later the same month, government witnesses said.

Strickland, a Washington native and graduate of Wilson High School in the District, started in the early 1970s as a small-time drug dealer, according to prosecutors.

Prosecutors charged that Strickland's drug-dealing career took off after he met Black, who allegedly introduced him in 1978 to Burns, the Miami lawyer.

Burns began selling Strickland cocaine at a wholesale price of about $49,000 a kilogram (2.2 pounds), according to prosecution evidence.