The Upper Burma Lawyers Network, which uses the old name for the country, said a canister taken from the scene after the protest in November had been examined by a laboratory in Thailand, The New York Times reported. A report by the Bangkok lab said phosphorus residue was found in the canister.
"We are confident that they used a munition that contained phosphorus," said U Thein Than Oo, the head of the network's legal committee. "They wanted to warn the entire population not to protest. They wanted to intimidate the people."
Ashin Tikhanyana, 64, a monk who was the most severely injured of the protesters, remains in intensive care in a Bangkok hospital. He was transferred to Thailand because he had been burned over 40 percent of his body and no hospital in Myanmar had the facilities to care for him.
"I saw a fireball beside me and I started to burn," he told the Times on Wednesday, describing the events of Nov. 29. "I was rolling on the ground to try to put it out."
Dr. Chatchai Pruksapong, a burn specialist caring for Tikhanyana, said the injuries were not caused by tear gas. He said they resemble those he has seen on soldiers injured by bombs while fighting rebels in southern Thailand.
A spokesman for President Thein Sein said he did not know if police used phosphorus.
Thein Sein heads the civilian government that took power in 2011 after years of military leadership. He was prime minister in the last years of the military junta, and critics say the new government is basically the same as the old one.