Jakarta – An Indonesian human rights group Monday criticized the country's anti-terror police for allegedly killing three people suspected of involvement in a deadly heist on Sumatra island.

Members of the Detachment 88 anti-terror unit, funded and trained by the United States and Australia, gunned down the three suspects during an operation in North Sumatra's Tanjung Balai area on Sunday and arrested 15 others in a separate location.

"We question whether the police did the right thing by killing the suspects," said Usman Hamid, chairman of Kontras, a leading human rights group. "Their action violated the rights of suspects," he said.

Police said the raids were part of an investigation into a robbery at an Indonesian branch of Malaysian bank CIMB last month in which a policeman was killed and two security guards wounded. The thieves made off with around 40,000 dollars in cash.

The involvement of Detachment 88 led some media to speculate that terror activities may have been involved.

National police spokesman Iskandar Hasan did said "We have not yet found a link to other things," such as terrorism, but that the possible link was "part of the investigation into the robbery."

The head of the National Counter-Terrorism Agency, Ansyaad Mbai, said militants had in the past been involved in robbery to raise funds for attacks. "We must not rule out that the robbery was linked to terrorism," he was quoted as saying by Detik.com news website.

Police have already come under fire for increased use of lethal force in dealing with terrorist suspects.

Indonesia has been hit by a series of attacks blamed on Islamic militants since 2000, notably in 2002 when 202 people were killed by bombings in Bali.