U.S. officials have acquired a tape recording of the torture of an American investigator that indicates Mexican police involvement in his murder, and have granted temporary asylum to 29 Mexicans allegedly threatened by drug traffickers linked to the case, according to federal government sources here.

The fast-breaking developments in the investigation of the year-old murder of U.S. Drug Enforcement Administration agent Enrique Camarena Salazar indicate continued American difficulty in securing Mexican government cooperation and a persistent threat to the lives of key witnesses in Mexico.

Camarena, a native of Calexico, Calif., who left a wife and three children, can be heard screaming on the tape, one source here said. According to a federal source in Washington, the tape contains a reference to a high official of the Mexican Federal Judicial Police who allegedly supervised the killing. Although not identified by name, U.S. officials think they know who the man is. He is believed to still be in Mexico.

The officials also said that Mexican authorities believe the tape contains the voice of Rene Martin Verdugo, a suspected drug trafficker in U.S. custody in San Diego after he was allegedly kidnaped by Mexican police friendly to the United States.

Mexican authorities, who first obtained the tape, were reportedly reluctant to turn it over to U.S. investigators because of the potentially embarrassing revelations. U.S. officials in Mexico and Washington have been bitterly critical of the slow pace of Mexican efforts to track down the killers of Camarena and Alfredo Zavala Avelar, a Mexican pilot employed by DEA who was kidnaped, tortured and killed at the same time.

American officials have been so distrustful of Mexican authorities that they have taken the unusual measure of granting temporary visas to six Mexicans and their families -- 29 people in all -- allegedly involved in last month's kidnaping of Verdugo.

U.S. officials have questioned Verdugo in the Camarena killing. While convinced that Verdugo was present during the torture -- a claim his attorney denies -- they say they do not know if he participated.

The tape recording was apparently made so the killers could refer back to whatever information they managed to extract from Camarena.

Verdugo, 34, was arrested near the border in Calexico Jan. 24 and charged in connection with a 1983 case involving the smuggling of about a ton of marijuana into Arizona and California. At a court hearing Jan. 29, Verdugo's attorney said Verdugo was abducted in Mexico by six Mexican men, driven to the border and shoved through a hole in the fence where U.S. marshals were waiting for him. The attorney, Howard Frank, said today that he believes the U.S. government paid the alleged kidnapers. A spokesman for the U.S. Marshals Service declined to comment on the alleged kidnaping case.

An official with the U.S. Immigration and Naturalization Service here confirmed Thursday that six men, including four members of the Mexican federal police, and their families had been granted special visas Feb. 2 allowing them to stay three to six months in the United States. A U.S. official said the action was taken after the men reported threats from Mexican drug traffickers unhappy about Verdugo's arrest.

The Los Angeles Times reported today that officials who have listened to the tape of Camarena's screams called them "shocking and helpless." Camarena, 37, was kidnaped in front of the U.S. consulate in Guadalajara, where he was based, on Feb. 7, 1985, and reportedly driven to the house of Rafael Caro Quintero, a drug trafficker now in a Mexico City prison. The Times said Camarena was beaten for several hours and killed about 8 p.m. Feb. 8 by blows from a metal rod, possibly a tire iron.

The bodies of Camarena and Zavala Avelar, the pilot, were buried in plastic bags at a ranch 70 miles southeast of Guadalajara, where they were found March 5.

A DEA spokesman in Washington declined to comment on what he said was his agency's ongoing investigation of Camarena's murder. A spokesman for the U.S. Marshals Service also declined comment.

An INS spokesman declined to comment on the reasons for the 29 temporary visas, but said that 14 of the people had been issued visitor visas valid until Aug. 2, and 15 have been allowed into the United States under another statute until May 2. The visas and permits can be renewed.

Several Mexicans have been jailed in connection with Camarena's murder, including some police officers, Caro Quintero and one other accused drug trafficker. Also in custody is Armando Pavon Reyes, a top federal police official who allegedly accepted a bribe to allow Caro Quintero to flee the country. The Mexican fugitive was later recaptured in Costa Rica.

Two other federal police commanders -- Alberto Arteaga Garcia and Miguel Inurreta -- have been questioned in connection with the Camarena case. U.S. officials say both men have been fired butremain in Mexico.