With a bundle of day lilies on one arm and her only surviving son at her side, Veronica de Negri told dozens of weeping friends yesterday that her oldest child was "murdered" in Chile because he "dared to search for justice."

"I am so proud of Rodrigo," she said in an emotional, faltering speech after arriving at National Airport from Santiago. "I was tortured and when I saw my boy I remembered that pain. I know how much my son suffered. Pinochet says he had a bomb, but I know the only bomb my son had was a camera."

The violent death last week of Rodrigo Rojas, 19, has drawn nationwide attention and put new pressure on the human rights policies of Chilean leader Gen. Augusto Pinochet.

Rojas, a Washington resident since his mother became an exile in 1975, died in Santiago of severe burns on July 6 after returning to Chile to learn about his native land. Friends and human rights advocates said that he and a companion were brutally beaten by a Chilean army squad that drenched them in gasoline and set them both on fire. Chilean military officials deny all allegations of impropriety.

The U.S. State Department has called for a full investigation of the incident. Citing "failures of many past investigations in Chile to bring justice to those responsible for violent crimes," the department yesterday dispatched Robert S. Gelbard, a deputy assistant secretary for inter-American affairs, to Santiago.

Sen. Jesse Helms (R-N.C.) arrived in Santiago Thursday and sharply criticized the State Department for pressuring Chile on investigating Rojas' death, saying that the United States "ought to understand that Chile is one of two countries in the entire Latin American area that resists communism."

In response, a State Department spokesman said yesterday, "We stand by our statements calling for a prompt, thorough and completely impartial inquiry into the Rojas death."

In addition, de Negri called for the Reagan administration to halt economic aid to Pinochet's government, saying the money will be used "to kill, to burn, to arrest and to persecute," and not to encourage democracy.

As she spoke, new details emerged of the incident in which her son was burned.

She said that she had spoken to many witnesses -- as have U.S. officials -- and they told her that her son and his companion, a young Chilean woman who remains in critical condition, were conscious when they were dumped in a ditch on the edge of Santiago.

"They crawled out of the ditch to seek help and they were like ghosts from another world," she said, repeating accounts she had received while in Santiago. "People wanted to help them, but they were so afraid."

Rojas had gone with a group of about 50 university students on July 2 to a shantytown to help rebuild houses that were destroyed by the military during a general strike.

Rojas graduated from Woodrow Wilson High School here this year and planned to spend several months in Chile taking photographs. His mother said yesterday that he was enraged by the suffering he saw.

"The government is trying to say my son was a terrorist; that is one of their cheap lies," she said. "My son was buried because he had a camera."

She said that she had to buy the medicine doctors used to treat her son at Santiago's Posta Central Hospital, and that each day that she was there she had to purchase diapers for him and change them herself.

"The day my son died the doctor called me asking for pills," she said. "When I arrived there without them a nurse said to me, 'I'm sorry, but you are supposed to supply the medicine.' "

She said that she had "deep admiration" for the doctors and nurses who treated her son and that she did not hold them responsible for his death.

Ariel Dorfman, a human rights activist who is close to Rojas' family, yesterday repeated a charge that police in Chile prevented Rojas from being transferred from Posta Central to a hospital that was better equipped to handle victims of serious burns.

"It is hard to imagine the brutality of this murder," said Dorfman. "He was a fine young man, an innocent who was murdered for trying to find his roots. But this is the history of dictatorships. Pinochet has become more and more brutal, and perhaps he has finally made a fatal mistake.

"I can't tell you," he continued, "how much I grieve that the name of that mistake was Rodrigo Rojas."