WARSAW, Poland (AP) _ Belarus has invited the United States to take part in the investigation of why an American balloon flying in an international air race was shot down, U.S. officials said today.

The bodies of the downed balloonists _ Alan Fraenckel, 55, and John Stuart-Jervis, 68, both of St. Croix, U.S. Virgin Islands _ were being flown by helicopter to the Belarussian capital of Minsk this afternoon, it said.

Meanwhile, two other American balloonists, forced to land by the Belarussian military and held without explanation, crossed into Poland today after being fined for not having visas.

Their balloon was forced down Tuesday in Belarus, the same day a military helicopter shot down the first American hydrogen balloon, killing Fraenckel and Stuart-Jervis.

J. Michael Wallace of Longmeadow, Mass., and Kevin Brielmann of Cheshire, Conn. arrived in Terespol, Poland, today. They telephoned the U.S. Embassy in Minsk to say they had arrived in Poland with a ground crew that had gone to pick them up. It was not clear where they were headed.

Belarussian border guards charged them $30 for not having visas, according to Susan Cleary of the U.S. Embassy in Warsaw.

A third crew of two American balloonists also forced down was released Wednesday and crossed into Poland that day.

``According to our information all Americans involved are out of Belarus,'' Cleary said today.

The U.S. embassy in Minsk said the Belarussian government has officially invited the United States to take part in the investigation of the downing of the first balloon.

Fraenckel and Stuart-Jervis were over rural western Belarus on Tuesday when a helicopter gunship opened fire from 100 yards away. Their balloon plunged 1 1/2 miles to the forest below. Two other American-manned balloons were then forced to land in the former Soviet republic.

Poland's defense minister, Zbigniew Okonski, responded angrily today to reported Belarussian accusations that Polish air traffic controllers failed to notify them of the approaching balloons.

``Belarus was informed by the Polish side about the flight of the balloons and we have a proof of this,'' Okonski told reporters.

In Belarus, Yuri Sivakov, a deputy head of the Belarussian government panel investigating the incident, told reporters the helicopter pilot that shot down the balloon thought it was unmanned.

``Everything was done in strict accordance to procedure,'' Sivakov said. ``The moral side of what happened is another thing.''

Many Belarussians were scornful of their government for the incident. The leading opposition daily, Svoboda (Freedom), said in a front-page commentary that the incident ``has left a black stain on Belarus' reputation.''

The government-controlled daily newspaper Sovietskaya Belarus blamed the military and scoffed at air defense officials for downing a balloon. ``The West wouldn't have chosen such an archaic craft for a spy mission,'' it said.

Valery Tzepkalo, the first deputy foreign minister, said his country was notified of the race in March. But he accused race organizers of ``creating risky situations'' by not providing enough information.

Belarus officials have also said the helicopter opened fire after Fraenckel and Stuart-Jervis failed to respond to radio calls warning them away from the military base.

Race officials say the crew's radio batteries might have been dead because they were nearing the end of the competition.

The balloonists were competing in the annual Gordon Bennett gas balloon race, a contest to see which balloon can fly the farthest. The balloons took off Saturday from Switzerland with no set course.