U.S. officials, critical of the pace of a Mexican inquiry into the kidnaping and murder of federal drug agent Enrique S. Camarena nearly a year ago, have convened a grand jury here to take testimony in the case, the Los Angeles Times has learned.

The grand jury proceeding, which has been under way for at least two months, underscores U.S. efforts to retain a strong role in the Camarena inquiry, regardless of what action Mexican authorities take.

Camarena and Alfredo Zavala Avelar, a Mexican pilot who had flown missions for the Drug Enforcement Administration, were kidnaped from separate locations in Guadalajara last Feb. 7. Their battered bodies were found March 5 in a shallow grave on a nearby ranch.

From the start, the case has strained U.S.-Mexico relations, at one point prompting American authorities to clamp down on vehicle traffic along the border.

A DEA spokesman told the Times the Mexican investigation is "moving frustratingly slowly. We're convinced the Mexican attorney general's office is involved in the investigation, but it's not moving as vigorously as we would prefer."

Justice Department and DEA spokesmen did not confirm that the grand jury had been convened. But sources familiar with the probe said that a special unit of DEA agents has been assigned to the inquiry, which is being overseen by the department's criminal division.