FORT STEWART, Ga. (AP) _ The Army suspended the firing of mortars at Fort Stewart while it determines what caused the explosion that killed two soldiers handling one of the weapons.

Officials from the Army Safety Center at Fort Rucker, Ala., were investigating Thursday's accident in Georgia and another accident the day before at Fort Sill, Okla., where an errant artillery shell killed three soldiers and wounded 24.

There was no relationship between the accidents, which involved different types of weapons, said Lt. Colonel Herbert Blanks, public affairs officer for the safety center.

The 4.2-inch mortar that exploded could have been faulty, or one of the victims might have made an error setting the timing fuse on the device, said Maj. Donald W. Keeling, a Fort Stewart spokesman.

''At this point, we really don't know what happened, but they were the ones handling the munitions,'' Keeling said.

A mortar is a short-barreled cannon that can be fired from a position on the ground or from fittings aboard a vehicle. Its shell contains shrapnel.

During training exercises, Keeling said, mortar tubes are loaded by two- person teams.

One team member sights the weapon and the other loads the explosive propellant and drops the round into the muzzle. The propellant is designed to detonate when the shell hits the bottom of the mortar tube.

It was not known at what point the shell exploded in Thursday's accident.

At Fort Sill, a four-man team of investigators picked their way through the accident scene, pulling shrapnel from trees and nearby buildings and poking at a 2-foot wide, 1-foot deep crater left by the stray shell.

Three types of howitzers - 105mm, 155mm and 8-inch guns - were being used in target practice at Fort Sill on Wednesday. Investigators must determine which gun the shell came from, and whether the firing error was human or mechanical, said Brig. Gen. Lou Hennies, director of Army safety for the Army Safety Center.

''If it's human, it could be in standards, in training, in leaders, in individuals or any combination thereof,'' Hennies said.''We're not in the blaming process ... for punitive measures. This is preventive.''

Post officials said the explosion occurred 700 to 1,000 meters beyond the target site. The shell landed about 10 yards from where a group of trainees had lined in formation, preparing to move to their next exercise.

The army identified those killed by the blast as Pvt. 1 Jimmy N. McCain, 21, of Troy, Ala.; Pvt. 1 Edward L. Zastrow, 18, of Lester Prairie, Minn., and Spec. Thomas P. Boyle, 25, of Ventura, Calif.

Staff Sgt. Scottie Harris of Rockmart, Ga., and Pvt. 1 David Saltsman of Dayton, Texas, were in critical condition early today, and Pvts. 1 David Adams of Vinemont, Ala., Troy Emmons of Hubbard, Neb., Charles McCrossen of Mason, Ohio, and David Mechem of Wichita, Kan., were in serious condition, officials said.

Seventeen other soldiers remained hospitalized, 15 in stable condition and two in good condition. One soldier was released Thursday, said Col. William Gonzales, commander of the Reynold Army Hospital. Gonzales said others probably would be released today.

The dead Fort Stewart soldiers are Pfc. Marvin Edmond, 22, of Fort Worth, Texas, and Spec. 4 Scott Reynolds, 27, of Graettinger, Iowa, said another base spokesman, Dean Wohlgemuth.