New York (CNN) -- Police and FBI agents finished searching 18 "spots of interest" in Long Island, New York on Thursday without uncovering additional evidence in their investigation of a possible serial killer.
"We didn't uncover anything that was noteworthy," said Nassau County Police spokesperson Kevin Smith. "Most of it was debris."
Their efforts are part of a probe into suspected serial killings that have turned up at least eight bodies -- four of them identified as prostitutes -- since December 2010.
New York State troopers alongside some 30 police officers from Long Island's Nassau and Suffolk counties hacked their way through the area's thick brush and bramble, 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," said Nassau County Acting Police Commissioner Thomas Krumpter.
But as of Thursday, authorities had not uncovered any further human remains.
Earlier this week, police discovered skeletal remains -- including a human skull -- prompting the widening search. Police have not said whether they are from different people, or if they could have come from the eight victims found previously.
Airplanes and helicopters with high-resolution technology are being used in the search effort, Suffolk County Police Commissioner Richard Dormer explained Wednesday. "
Police also are using 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.
"Canine teams have been of limited assistance," said Detective Vincent Garcia. "The problem with this brush is it's so thick dogs won't even go in."
Meanwhile, medical examiner investigators are trying to determine the age and sex of the victims from the remains, and how long they were at the locations before they were discovered.
"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 Ellen Borakove, a spokeswoman for the New York City medical examiner's office.
"Hopefully we're going to get some IDs and we'll find out who these people are, and then the families will know," she added.
The remains found Monday in a Nassau County wildlife preserve were wrapped in a "black plastic product" similar to a plastic bag, a law enforcement source said Wednesday.
The remains "appeared to be part of a lower extremity," the source said.
But police say the remains are not connected to the bodies of four prostitutes found near Atlantic City, New Jersey, in 2006.
"The indications we have right now is that there's no connection between the Atlantic City case and the Suffolk County case," Dormer said.
"There are other items connected with the two cases" that would indicate the same killer was not involved in the slayings, he said.
Dormer did not provide additional details, but said Suffolk County police officers have been in contact with their Atlantic City counterparts since the early stages of the Long Island investigation.
Federal agents first joined police in the investigation in December after the first four bodies were found.
That discovery on a quarter-mile stretch of Oak Beach indicates that "they were dumped there by the same person or persons," Dormer told reporters at the time. "It's too coincidental that there were four bodies in the same location."
They were all found stuffed in bushes along an isolated strip of waterfront property.
The grisly find occurred as police searched for Shannan Gilbert, 24, who was last seen alive in May 2010 in the Gilgo Beach area.
Gilbert's sisters said Shannan was an escort who was visiting a client. They said she ran from the man's house and called 911, claiming that someone was trying to hurt her.
A man who lives a few miles from where the remains were found told CNN that a young woman who "looked like she was partying all night" came to his door at about 5 a.m. on May 1.
"She was screaming 'Help me, help me!' and said somebody was chasing her," said Gustav Coletti, who lives in the town of Oak Beach.
He described the woman as a "young, light-brown-haired person in her early 20s."
Coletti said he called 911 after opening his front door to the woman, but she fled, eventually hiding under his boat, which was elevated and parked on his lawn.
Moments later, Coletti said, an "Asian-looking man" man driving a dark-colored sport utility vehicle pulled up alongside his home.
"Things got out of hand at a party and I'm just looking for her," Coletti quoted the man as saying.
When Coletti told the man that he had called police, the man said he "shouldn't have done that." The woman then fled toward the beach and the driver pursued her, Coletti said.
But he told CNN it took police more than four months to question him about the incident, an allegation that police deny.
Coletti later told CNN that he believes the woman he saw that night was Gilbert.
CNN can not independently verify that claim.
"I have no idea when the detective spoke to whomever you are talking about," Dormer told CNN. "If you want to see somebody after this press conference we'll certainly follow up on that."
Police later clarified that they spoke with Coletti on May 1 after he made the 911 call and several times since.
Pressed for the dates of the police department's conversations with Coletti and the length of time between them, they revealed that they obtained a statement from Coletti in June but provided no further specifics.
"They never got a statement from me in June," Coletti told CNN.
Gilbert's body has not been among the remains identified.
CNN's Allan Chernoff and Susan Candiotti contributed to this report