New York (CNN) -- As they worked to determine if new bones found off Long Island, New York, were human -- and possibly those of victims of a suspected serial killer believed to have targeted sex workers -- tests were underway Monday to match up identities with at least four other sets of discovered remains.
Police have previously identified four victims found on Oak Beach in December 2010 as women who advertised prostitution services on websites such as Craigslist. Now, "the DNA process is underway" on at least five other sets of remains found in and around nearby beaches in the time since, New York City medical examiner's office spokeswoman Ellen Borakove said Monday.
Investigators are trying to determine the identities of these victims, while looking for clues about their killer or killers and any link between all the deaths.
The remains include a skull and bones found as recently as last week in Nassau County, as well as a bone fragment of unknown origin found by divers off the Oak Beach area last Friday.
New York City's medical examiner office -- the largest non-profit DNA lab in the United States, according to Borakove -- is doing the analysis, and will then turn over the information to investigators in Suffolk and Nassau counties and the New York state police. Experts will extract DNA from the remains to create a profile, which can be used by investigators.
"We're going back to some of the unidentified people in our system to try to see if we can identify them, so we can notify loved ones if we have their remains," said Borakove. "Hopefully ... we'll find out who these people are."
Meanwhile, the search for more clues continued Monday.
FBI agents flew a Blackhawk helicopter over area beaches using high-tech equipment to photograph areas that have been previously examined and searched from the ground. The flyovers were finished by Monday evening.
New York State troopers and some 30 police officers from Long Island's Nassau and Suffolk counties hacked their way through the area's thick brush and bramble last week, using guidance from a helicopter overhead and, in some cases, using chainsaws to cut through the thick foliage.
Aviation teams that flew over parts of Long Island had "identified a significant number of items that aren't natural for the area," Nassau County Acting Police Commissioner Thomas Krumpter said last Thursday.
Last week, police discovered skeletal remains -- including the human skull -- prompting the wider search. Police have not said whether the remains are from different people or if they could have come from the eight bodies found previously.
Police have used canine search units to scour the beaches and diver teams to search the waterways on the north side of the barrier island.
But they have met obstacles, such as thick brush that canine teams won't go in.
On April 11, remains were found in a Nassau County wildlife preserve, a law enforcement official said. The remains were wrapped in a "black plastic product" similar to a plastic bag, a law enforcement source said.
The remains "appeared to be part of a lower extremity," the source said.
Medical examiner investigators have been trying to determine the age and sex of the people whose remains were found, as well as how long they were at the locations before they were discovered.
A lawyer for the mother of 24-year-old Melissa Barthelemy, who police have identified as one of the victims, said that a man -- using the woman's cell phone -- made taunting phone calls and claimed to have killed the woman.
The lawyer, Steve Cohen, said on one August 26, 2009, call, the man said, "Do you think you'll ever see her again? You won't. I killed her."
Police have not publicly identified any suspects in the case, nor offered a profile of the person or persons they believe may have been responsible for the multiple deaths.
CNN's Susan Candiotti and Ross Levitt contributed to this report.