JACKSONVILLE, Fla. (AP) _ Carlos Lehder Rivas' brothers have threatened the life of a witness who may testify in reputed Colombian drug kingpin's cocaine-smuggling trial, officials said.

Lehder, 38, and Jack Carlton Reed, 57, are on trial on charges they conspired to smuggle some 3.3 tons of cocaine into the United States via Norman's Cay in the Bahamas.

U.S. Attorney Robert W. Merkle said Tuesday he had received reports from Colombia that Carlos Toro's relatives had been contacted by Lehder's brothers and told he shouldn't testify.

Also, on Monday an anonymous male caller telephoned Florida Publishing Co., which publishes the Jacksonville Journal and The Florida Times-Union, and said somebody ''would do away with'' Toro before he can testify.

Toro, a boyhood friend of Lehder, may be called to testify about smuggling some 2 tons of cocaine for Lehder through Andros Island, Bahamas, in 1983 and 1984.

Merkle refused to discuss any possible investigation into the incidents by the U.S. Marshal's Service.

Wendell Welman, a criminal investigator for the Internal Revenue Service, will continue his testimony today about financial records of Lehder's companies managed by Guardian Trust Ltd. of Nassau.

On Tuesday, Welman read a letter in the records of one of the companies owned by Lehder that told Bahamian officials that Lehder planned a massive remodeling on the island of Norman's Cay.

On April 20, 1979, a letter was written to Clement Maynard, director of tourism for the Bahamas, telling him that Lehder had bought a large part of Norman's Cay. It tells of his plan to close public facilities while they were being remodeled ''because of the poor state of repairs.''

The letter was written by Ian C. Davidson of Guardian Trust Ltd. on behalf of International Dutch Resources, one of Lehder's companies.

The letter said Lehder's company planned to file documents for permits to lengthen the airstrip, expand the yacht basin, build new staff quarters and a new store and possibly build a new restaurant and hotel on the island.

The letter supports defense contentions that Lehder planned to convert Norman's Cay into a tourist resort.

Prosecutors have contended that Lehder closed Norman's Cay to the public to protect the island as his cocaine-smuggling haven.

Welman has been testifying in great detail about documents relating to six of Lehder's companies.

At times, members of the jury and spectators have become drowsy, prompting U.S. District Judge Howell W. Melton to issue an instruction to the jury.

''If you are becoming drowsy, let me know if you feel you have a problem,'' said the judge, who told jurors he would then call a short break.