The bodies of three men and a woman killed by gunshot wounds were discovered near the farming village of Mahavilachchiya in the Anuradhapura district just south of rebel-controlled territory in the north, said a defense ministry official.
The army and police were searching the area for the rebels responsible for the attack, said the official, who spoke on condition of anonymity because he was not authorized to speak to the media.
Tamil Tiger spokesman Rasiah Ilanthirayan did not answer repeated calls from The Associated Press seeking comment. The rebels have fought the government since 1983 to create an independent homeland for ethnic minority Tamils after decades of discrimination from governments dominated by the Sinhalese majority.
Mahavilachchiya lies on the edge of Wilpattu National Park where four soldiers and four wildlife department employees were killed in a rebel attack in March.
The park, which borders the conflict zone, was closed after fighting erupted in the 1980s, but was partly reopened after a 2002 cease-fire.
A spike in violence since December 2005 has killed more than 5,000 people. More than 70,000 people have been killed since the conflict began.