Human remains found at World Trade Center site as investigators sift through debris left by 9/11 terrorist attack in New York

Families continue to be frustrated at over 1,000 victims left unidentified and progress of excavation

Investigators began sifting through debris from the World Trade Center site on Monday (AFP/Getty Images)

The possible remains of two people have been found among debris cleared from the World Trade Center in New York, over 11 years after the terrorist attack that killed 2,750 people.

Investigators began sifting through rubble recovered from the site for the first time in three years on Monday, with crews able to dig up parts of the site that were previously inaccessible to workers.

New York's last sifting effort ended in 2010, but now 60 truckloads of debris is being transported to a park built on top of a former landfill site on Staten Island, where investigators will attempt to identify any possible remains over the next 10 weeks.

The medical examiner confirmed that two potential human remains were found on Monday.

So far, only 1,634 of the 2,750 killed in the attacks have been identified, and 9,000 pieces of human remains recovered from the site remain unidentified because they are too degraded to match victims by DNA identification.

The remains are stored at an undisclosed location monitored by the medical examiner's office and will eventually be transferred to a subterranean chamber at the National September 11 Memorial & Museum.

Jim Riches pulled his firefighter son's mangled body out of the rubble at the World Trade Center, but the phone calls still filtered in years afterward. The city kept finding more pieces of his son.

He told the Associated Press: "They'll call you and they'll tell you, 'We found a shin bone.' Or: 'We found an arm bone.' We held them all together and then we put them in the cemetery."

All remains collected are stored at an undisclosed location monitored by the chief medical examiner's office and will eventually be moved to an underground chamber at the National September 11 Memorial & Museum.

But the victims of some families have expressed frustration at the time it has taken to uncover and go through more debris.

"Quite frankly, they should've excavated this and searched it 12 years ago," said Diane Horning, whose son, Matthew, died in the attacks. "Instead, they built service roads and construction roads and were more worried about the building and the tourism than they were about the human remains."

Others, who know the possibility of identifying their loved ones is near impossible, are more positive about the project to uncover more remains.

Charles G Wolf was pleased to hear about the renewed search, though he believes that his wife, Katherine, was vapourised during the attack. Investigators have never found her remains.

Years ago, it bothered him that he had no grave to visit. Wolf said the opening of the September 11 memorial has filled a hole in his heart, but he'll never have closure.

"You heal. You carry on," he said. "It's not closure."