The Sri Lankan defence ministry has announced an investigation into the killing of five young Tamils in the eastern town of Trincomalee.
Ceasefire monitors said the men had been shot in the head.
Tamil groups blame security forces but they deny any involvement. Two survivors have said their attackers wore military fatigues.
The killings come amid worsening violence in the north and east linked to Sri Lanka's ethnic conflict.
In Trincomalee on Wednesday tensions were high and shops closed in protest at the killings.
They followed a series of mine attacks blamed on Tamil Tiger rebels in December in which more than 40 military personnel died.
The Tigers denied responsibility, but the BBC's Dumeetha Luthra in Colombo says few in Sri Lanka believe them.
She says some observers believe the killing of the Tamils in Trincomalee could have been revenge.
Concern is growing that the country may be sliding back to war after years of conflict in which more than 64,000 people died.
Sri Lankan Foreign Minister Mangala Samaraweera is in Washington for talks. He meets US Secretary of State Condoleezza Rice on Thursday.