The mother of the 19-year-old District resident who died of burns in Santiago on July 6 cast new doubts yesterday on the Chilean armed forces' account of her son's death and asked the U.S. government to impose economic sanctions against Chile's leader, Gen. Augusto Pinochet.

Veronica de Negri, at a news conference here, accused the Pinochet government of trying to cover up the facts of the case by destroying key pieces of evidence and terrorizing eyewitnesses.

"Until the guilty are judged and punished, I will not rest," said de Negri, who returned a week ago from Chile where she attended her son's funeral.

The events leading to Rodrigo Rojas' death have been a source of dispute since he and a Chilean companion, Carmen Gloria Quintana, were discovered burned and beaten four miles outside of Santiago. Witnesses have said the two were doused with gasoline, set afire and then transported and abandoned by military officers who had tried to disperse an antigovernment demonstration.

The Chilean Army initially denied that any of its members were in the area of the demonstration. Last Friday, however, 25 soldiers were arrested and held for questioning in the case. De Negri expressed cautious optimism about the arrests yesterday, but said she was still receiving conflicting reports from the Chilean government.

Among the inconsistencies she cited was the claim that Rojas and Quintana were burned when a container of flammable liquid they were carrying ignited and that soldiers had extinguished the flames with blankets.

"If that is so, why weren't they taken to a hospital instead of appearing four miles away in Quilicura, on the other side of Santiago?" de Negri asked.

De Negri told reporters that the police in Quilicura had dressed her son in a parka, burned only on the inside, to confirm the soldiers' account. But according to a Catholic priest who attended the demonstration, Rojas had been wearing a navy blue overcoat that was destroyed when he was set on fire, de Negri said. She added that her efforts to recover her son's clothing and cameras had been unsuccessful and that members of her family and witnesses in Chile had been receiving death threats.

Since 1981, the U.S. government has voted to provide more than $2.1 billon in loans to Chile from the World Bank and the Inter-American Development Bank.

Recently, Elliot Abrams, assistant secretary of state for inter-American affairs, said that while the U.S. government opposed the human rights record of Pinochet's 13-year-old military rule, it did not support imposing economic sanctions.