"We have collected 80 skeletal remains and removed them for safe keeping," said Dhananjaya Waidyaratne, a judicial medical officer.
Excavations resumed on Monday after a short break since the first four skeletal remains were discovered on December 21 by construction workers in Thirukatheeswaram area of Mannar district, Waidyaratne said.
After a magisterial inquiry, forensic medicine officials were deployed in the area.
Officials had earlier said women and children were among those buried in the grave.
Officials said further tests are needed to establish how and when the people died.
Tamil leaders have said the victims could be members of the local Tamil community.
Mannar, which has a sizable Tamil population, had witnessed many battles between government troops and the LTTE during the civil war.
This was the first discovery of a mass grave in the northeast since the army crushed Tamil Tiger rebels in 2009 following a decades-long war for a separate homeland for ethnic minority Tamils.
The Sri Lankan government has ruled out the possibility that its soldiers could have been involved in the killing of those found in the grave, saying Mannar had long been a Tamil rebel stronghold.
"With regard to the recovery of skeletal remains in Mannar it has been revealed that the area had been occupied by the LTTE for 30 years except during the period 1988/89 when it was occupied by the Indian Peace Keeping Force. Till the area was liberated in 2008 it was not under the control of the government of Sri Lanka," an official statement said.
The mass grave figured in the government's response to United Nations Human Rights chief Navy Pillay's report on Sri Lanka which is expected to be submitted at the UN Human Rights Council's session in March.
The report is being seen as a preamble to the next US-backed resolution against Sri Lanka on alleged rights abuses.
Another mass grave found in the central district of Matale is also being investigated.
The opposition JVP or the People's Liberation Front has claimed that the bodies belonged to its cadres who were allegedly executed during a crackdown by the state during 1987-90.
Image: A site of the civil war in northern Sri Lanka ' Photograph: Reuters
How India secretly helped Lanka destroy the LTTE
'Tamil separatism may resurface in Lanka, but not LTTE'
How the Lankan army crushed the LTTE
Sri Lanka celebrates LTTE's end
Why SL's 'lies' on death of Prabhakaran's son may soon get exposed