An American drug agent found dead in Mexico was killed by a blow to the head with a blunt instrument and a Mexican pilot found dead with him apparently was buried alive, agents of the Drug Enforcement Administration said yesterday.

Secretary of State George P. Shultz issued a statement denouncing "these sickening murders" and vowing to step up the international war against drugs.

Earlier yesterday, Shultz indicated at a Senate hearing that the United States has not ruled out retaliatory action for the kidnapings and murders of DEA agent Enrique Camarena Salazar and Mexican pilot Alfredo Zavala Avelar, who sometimes flew DEA missions.

"Our level of tolerance has been exceeded by these events," he said. "But exactly what we will do about it is something we have to work out."

White House spokesman Larry Speakes said: "This is a tragic situation that we deeply regret," adding that he was certain President Reagan would be in touch with the agent's family.

On Wednesday night, shortly after Mexico told the United States the bodies had been found, DEA Administrator Jack Lawn repeated his agency's criticism of the Mexican government's "initial inertia" in response to Camarena's abduction Feb. 7.

A U.S. Air Force jet is scheduled to fly Camarena's body from Guadalajara to San Diego this morning. A motorcade of U.S. officials is to escort it to Camarena's home in Calixico, Calif. Camarena was married and had three children.

The pilot will be buried in Mexico, the DEA said.

Camarena, 37, a Mexican-born naturalized American with nearly 11 years' experience at the DEA, was abducted as he left his Guadalajara office to have lunch with his wife. An eyewitness told DEA investigators that Camarena was grabbed by four armed men and thrown into a car. The pilot was abducted the same day in a different section of the city.

The bodies were discovered by a boy at 4 a.m. Wednesday near the city of Zamora, 60 miles southeast of Guadalajara, the DEA said. Officials said they were wrapped in plastic and had been "dumped on a roadside" near a ranch where a reported shootout on Saturday between Mexican federal police and drug traffickers left five persons dead: a police officer and a man and his wife and two sons who lived on the ranch. Two women were arrested after the shootout.

The boy notified local authorities, who summoned Mexican federal judicial police, DEA spokesman Bob Feldkamp said. They arrived Wednesday and examined the bodies, he said, and then "went to their office in Guadalajara, called DEA and took us back to the site with them."

The bodies had been buried elsewhere, then exhumed and dumped near the ranch, DEA agents said.

They were taken to a Red Cross Hospital in Zamora. Two Red Cross physicians examined the badly decomposed bodies, and the U.S. Embassy in Mexico City cabled their findings to the State Department. They said the left side of the agent's skull had been crushed by a severe blow and other bones were broken, according to DEA sources. The pilot, who also had several broken bones, apparently died of suffocation, the doctors reported.

The bodies were taken to a morgue in Guadalajara and were examined yesterday by a U.S. military pathologist and an FBI identification specialist, who confirmed the Red Cross physicians' findings, the DEA said. The FBI specialist positively identified Camarena from dental charts, the DEA said.