The last of a group of 3,200 American soldiers sent to Honduras left for home Thursday as a special U.N. team confirmed Washington's claims that Nicaraguan forces crossed into Honduras in pursuit of the U.S.-backed Contra rebels, officials said.

The U.N. findings vindicated Washington's claims of a Sandinista military incursion and came as a blow to Nicaraguan President Daniel Ortega, who had repeatedly denied any violation of Honduran territory and had requested the investigation to counter the U.S. accusations.President Reagan ordered the 3,200 American troops to Honduras in a show of support for President Jose Azcona following the incursion, the last Nicaraguan offensive against the U.S.-supported rebels before the two sides signed a cease-fire in their 7-year-old war.

A U.S. Embassy spokesman said the last 80 American soldiers left Honduras aboard a C-141 transport jet Thursday morning for 82nd Airborne Division headquarters at Fort Bragg, N.C. Units of the 7th Infantry Division returned earlier to Fort Ord, Calif.

The jet departed at 8 a.m. from Palmerola Air Base, 38 miles north of Tegucigalpa. In Washington, Pentagon officials estimated the cost of the 10-day exercise at between $4 million and $5 million.

In a report to U.N. Secretary-General Javier Perez de Cuellar in New York, investigators said "it was acknowledged" that the Nicaraguan forces penetrated Honduran territory by upwards of two miles.

"At the peak of the operation as many as 4,200 Nicaraguan forces had been deployed in the area, but by the time the mission visited the area they were reportedly down to about 1,000," said the report, which was made available to United Press International.

It said the Nicaraguan offensive against the Contras lasted from March 3 to March 20 and was an operation designed to destroy rebel sanctuaries along the northern border with Honduras.

The four-man team, headed by Brazilian Gilberto Schlittler, was sent to Nicaragua March 22 at Ortega's request and returned to New York March 27. Honduras refused permission for the team to visit its territory.

The report said the team met with Ortega and was briefed by Nicaraguan military officials.

U.S. officials said Nicaraguan troops chasing the rebels entered Honduras in an effort to destroy the Contras and their bases, where much-needed supplies had been collected to carry the fighters through a period in which they were being denied U.S. military aid. Ortega had denied the troops entered Honduras.

The Reagan administration rushed in the soldiers March 17-18 after a plea for aid from Azcona. Administration and congressional sources have told UPI the United States pressured Azcona to ask for help to justify the show of force.

In Washington, Pentagon spokesman Dan Howard said the cost of the 10 day operation is estimated at between $4 million and $5 million. He said all of the combat troops would be back on U.S. soil Thursday and the few remaining support troops would return Friday.