Navy Secretary John F. Lehman Jr., who strongly defended the Navy's decision to stop an investigation of an admiral accused of taking weapons captured in Grenada as souvenirs, is trying to block a move to give greater autonomy to the service's criminal investigative agency, according to Defense Department officials.

Pentagon Inspector General Joseph H. Sherick urged Defense Secretary Caspar W. Weinberger last month to allow the Naval Investigative Service to operate without approval from Navy brass.

The investigative service's independence came into question because of news reports that the investigation of Vice Adm. Joseph Metcalf III was killed by his superiors after they deferred the case to Metcalf. The admiral acknowledged to investigators that he illegally brought back 12 Soviet-made firearms from Grenada in November 1983, then asked, "What is there to investigate?" according to a Navy document.

Lehman, who says he stopped the Metcalf probe personally because "I never thought there was anything left to be done," has interceded with Weinberger to preserve command control over the NIS, according to Pentagon officials.

Lehman, the officials said, has argued that an independent investigating agency would undercut authority of local commanders. To strengthen the process, however, he reportedly has suggested that his office be given the power of final approval in cases where local commanders stop a NIS probe.

Sherick has countered that NIS should be free to set the terms of its work without interference from the Navy secretary, who theoretically could become the target of investigators, officials said.

"It puts the secretary of the Navy in a politically untenable position," said one source. "If the rule doesn't apply to him, it shouldn't apply to the Navy."

Weinberger is weighing the options at the request of Rep. Jack Brooks (D-Tex.), who said in a letter last month that the Metcalf case "creates an appearance that investigators conveniently look the other way when an investigation involves high-ranking military officers."

Metcalf, who commanded the U.S. invasion of Grenada in October 1983, received a nonpunitive "caution" while several lower-ranking soldiers and Marines were ordered to jail for up to three years for smuggling and, in some cases, selling the AK47s.

The handling of Metcalf's case stirred controversy in the Pentagon even before it became public in February.

Sherick, whose office acts as an independent Pentagon watchdog, wrote a memo to Lehman in January 1984 questioning why "the authority to request investigation of the matter by the Naval Investigative Service was delegated to the subject of the investigation."

In a later interview, Lehman explained that he felt the case was closed because Metcalf admitted wrongdoing and received a warning from his superiors.

Lehman could not be reached for comment yesterday.