Along with all Guatemalans we want the threats to stop

US Ambassador Prudence Bushnell

"The threats were addressed to 11 people specifically," said the group's president Fredy Peccerelli. These 11 were singled out as being "the first to die".

The group has been excavating clandestine cemeteries, looking for the remains of 200,000 people believed to have died in the conflict between left-wing rebels and military governments, between 1960 and 1996.

Ten copies of the letter containing the threats were delivered to an anthropologist's home, saying that the "dead should be left for dead", otherwise the group would "regret they had been born".

Mr Peccerelli said the message had arrived on 21 February, the day his group was marking the Day of Victims' Dignity, an unofficial holiday honouring those who died or disappeared during the civil war.

The foundation filed a complaint to the government on Monday, and Interior Minister Eduardo Arevalo has promised to provide extra security for the group's headquarters in Guatemala City.

The US Ambassador to Guatemala, Prudence Bushnell, described the threats as very troubling.

"Along with all Guatemalans we want the threats to stop," she said.

Identifying victims

Since 1992, the Forensic Anthropology Foundation has excavated 191 clandestine cemeteries throughout the country, uncovering more than 2,000 massacre victims.

Relatives want to identify and bury their war dead

Their aim is to identify corpses, give them proper burials and find clues to bring the perpetrators to justice.

There have been an increased number of death threats against human rights groups in Guatemala in the past few years.

Some of these groups are aiming to take former military commanders to court on charges of genocide - including General Efrain Rios Montt, who ruled Guatemala from 1982 to 1983, and currently presides over Congress.