LA PAZ, Bolivia (AP) _ Authorities have arrested members of a Marxist group that killed two U.S. missionaries and bombed the motorcade of ex-U.S. Secretary of State George P. Shultz during a visit last year, the government said.

''In recent days, we apprehended three important members of the group Zarate Willka,'' one of whom confessed to the motorcade attack, Interior Minister Eduardo Perez Beltran told a news conference Thursday night.

The Armed Liberation Front of Zarate Willka claimed responsibility for the killings in May of Todd Ray Wilson and Jeffrey Brent Ball, both 20-year-old Mormon missionaries from Utah.

Police found a copy of the group's message to the media on the killings when they searched the residence of brothers Felix and Nestor Encinas Laguna, the interior minister said. Authorities also found explosives and detonators there. Felix Encinas Laguna, a sociology student, and his brother, a member of the technical faculty at a local university, were at large.

Perez Beltran said sociology student Constantino Uujra, alias Pancho, was arrested. The interior minister said the suspect confessed to the attack in August last year during Shultz's visit.

In the attack, an assailant threw an explosive device near the motorcade taking Shultz from the airport to the city center. The blast broke the windows of some vehicles but there were no injuries.

Perez Beltran said authorities also arrested the group's ideologue, physician Gabriel Rojas, and medical student Telma Salazar, identified as another group member.

Ball, of Coalville, Utah, and Wilson, of Wellington, Utah, were gunned down late May 24 as they returned to their apartment in downtown La Paz. The two were machine gunned by occupants of a small yellow car.

Zarate Willka is named after a leader of a civil uprising in 1890 against a law permitting the sale of communal land used by peasants.