Sri Lanka's army has denied involvement in the killing of five Tamil youths in the eastern town of Trincomalee.
Two survivors of Monday's attack said men in military fatigues had thrown a grenade at the group before dragging them into a truck and shooting them.
Ceasefire monitors said the killings resembled executions. "All five were shot in the head," a spokeswoman said.
It is the latest in a string of deadly attacks in the north and east linked to Sri Lanka's ethnic conflict.
The two students who survived Monday's attack told a post mortem inquest they had gathered at a popular meeting point near the beach in Trincomalee, about 250km (150 miles) north-east of Colombo, when the truck pulled up.
They could not confirm the identity of their attackers.
The army has ruled out an inquiry into the killings, saying the police are conducting one.
A statement on the army's website said the youths died when a hand grenade in their possession went off by accident. It accused them of plotting to attack security forces.
Tamil Tiger rebels said Sri Lankan military special forces had killed the youths.
The violence comes after a string of recent attacks on soldiers, in which more than 40 troops have died in the north and east.
The government blames the attacks on the Tamil Tigers. A ceasefire between the two sides is under severe strain.
The recent violence is the worst since the truce was agreed four years ago, after two decades of conflict had claimed more than 64,000 lives.
The Tigers want autonomy for minority Tamils in the north and east.