MEXICO CITY — The death toll in the crash of a Mexican military helicopter shot down Friday by members of a drug gang climbed to six Monday after tests of remains at the scene determined that they were of three soldiers initially reported as missing.

The helicopter, a Eurocopter EC725 Cougar, was participating in a sweep to weaken the gang, the Jalisco New Generation, which has emerged as a serious threat in southwestern Mexico.

Members of the gang fired a rocket-propelled grenade, a testament to the firepower the group has attained, bringing down the helicopter in a rural area of Jalisco State, according to Monte Alejandro Rubido, the federal security commissioner.

Mr. Rubido said Monday that the shooting down of a helicopter of that size by a drug gang was unprecedented in the country and that such a fatal attack on military aircraft was rare. It will lead the authorities to redouble their efforts to cripple the gang, he said.