A French relief agency, Action Against Hunger, says two more of its workers have been found dead in the Sri Lankan town of Muttur.
There has been international outrage over the killings
On Sunday, 15 aid workers were found dead in their compound lying face down and shot at close range.
There has been widespread international outrage at the killings, which came as government and rebel forces fought over a water dispute.
Both sides have accused each other of the killing of the aid workers.
The two new bodies were found in a car - they had apparently been killed while trying to flee the scene of the attack on the aid group's compound.
Action Against Hunger has suspended all its work in the area and says it is waiting for the results of a post mortem.
The Sri Lankan government has promised an independent investigation into the killings of the workers - 13 men and four women.
Separately on Tuesday, the Sri Lankan military said suspected Tamil Tiger rebels had ambushed a government patrol near an air force base in the north-east, killing one person and injuring two others.
Shock at killings
The Paris-based Action Against Hunger group (AAH) said that the 15 employees were shot over the weekend in Muttur, where they helped survivors of the December 2004 tsunami and people affected by violence in the country.
AAH Director-General Benoit Miribel said the charity had not suffered such a loss in its 25 years of existence.
It is now reviewing its presence in the country.
Mr Miribel said the group wanted to send a team to the area but was prevented from doing so by soldiers.
The killings have been strongly criticised by the international community.
Amnesty International pressed the government to seek international help in its investigation.
"While we welcome the government's statement that this investigation will be treated as a matter of high priority, we are concerned that other high profile cases of killings and disappearances remain unresolved months and even years after the crime," Natalie Hill, deputy Asia director of the watch group, said.
Fighting between the army and Tamil Tiger rebels erupted in the Muttur area more than two weeks ago, after the rebels cut the water supply to mostly Sinhalese villages.