WOLF CREEK, Mont. (AP) _ Police fanned out across mountainous terrain along a Missouri River reservoir today, searching for up to four snipers who slipped away after firing a barrage of shots at officers and wounding two campers.

Lewis and Clark County Sheriff Chuck O'Reilly told reporters that the snipers were still at large and heavily armed with large-caliber and automatic weapons.

''Right now, we are sweeping cabins and residences,'' O'Reilly said. Aircraft with heat sensing devices also were used, but ''we've been unable to locate them.''

More than 60 officers, including officers from the FBI and several federal agencies, were involved in the manhunt, the sheriff said, but the snipers apparently got away during the night.

''That's awful big, high country up there,'' said Sheriff's Sgt. Al Brockway.

Shortly after 8 a.m. today, reporters heard what sounded like automatic weapons followed by bursts from several rifles.

But O'Reilly said he could not confirm that any shots were fired and said law enforcement officers had not fired any shots.

The sheriff said at least three individuals, two men and a women, had been seen, and there might be a fourth person, possibly a woman. Two of the snipers were believed to be suspects in a double slaying in Colorado.

A campground and scattered rural residences were evacuated after gunfire broke out Monday at the site overlooking Holter Lake, a reservoir on the Missouri River midway between Helena and Great Falls.

''As we started a sweep of the mountainside, literally all hell broke loose,'' O'Reilly said Monday. ''They must have had scads of ammunition.''

Police said they believed there were four snipers because gunfire came from four directions Monday. O' Reilly said the snipers used six or seven types of weapons, including an automatic weapon, and possibly a machine gun.

Montana Highway Patrol officers, sheriff's deputies and a Helena police SWAT team had been kept at bay by sporadic gunfire.

''They're shooting at boats on the lake, campers, anyone who comes by,'' Carl McLaughlin, who lives near Holter Lake, said Monday evening. ''I heard seven consecutive shots.''

Police cordoned off the nearby towns of Craig and Wolf Creek.

Two passers-by were treated for minor wounds from shrapnel or flying glass after gunfire struck their car, O'Reilly said. One of the snipers also may be injured, he said.

Two of the snipers were believed to be suspects in a double homicide in Adams County, Colo., police said. The suspects were identified as Michael Brandt and David Lund, who are believed to be have escaped from a California prison late in 1985.

The shooting started at mid-afternoon Monday, shortly after a Highway Patrol officer began investigating an abandoned van with Colorado plates that rolled over near Holter Lake, police said.

The van was being sought in connection with the shooting deaths of Grace Kuamoo, 73, and Russell Schwartzmiller, 48, patrol spokesman Randy Yaeger said. Their bodies were found Saturday.

The suspects worked for Schwartzmiller for about a year under aliases and were last seen Thursday on his property, Adams County Sheriff's Capt. Carl Blesch said.

Their identities won't be confirmed until fingerprints can be compared with those from the California penal system, he said.

The two men were found in the Juniper Bay Campground, where they began shooting at officers, Yeager said.

Police pulled back as darkness fell and concentrated on evacuating an unknown number of residents and campers from the lake, Brockway said.

Most of the homes are used mainly on weekends or during the summer.