New York (CNN) -- Police are examining what appears to be a human skull found on the outskirts of a bird sanctuary in Long Island, New York, potentially the ninth set of remains discovered in the probe of a suspected serial killer.

Lt. Kevin Smith of the Nassau County Police Department said the skull was found Monday around 3:30 p.m. in the town of Oyster Bay. About four hours earlier, a New York state police officer and his cadaver dog spotted another set of remains about a mile and a half away, and authorities worked to determine whether those remains are human.

Authorities could continue their search on Tuesday.

These discoveries come on the first day that authorities probing the deaths of several young women searched for evidence in and around Nassau County. Since December, eight sets of human remains have been found in Suffolk County, just to the east of Nassau County and roughly five miles from the sites of Monday's discoveries.

"It's just been very startling, all of it has been," Smith told reporters Monday. "It's just amazing that we're finding these things. Obviously, we have a lot of work to do."

Authorities are working on the presumption that a single person is responsible for the women's killings. The four victims who have been identified thus far all advertised prostitution services on websites such as Craigslist, police have said.

"Collectively, we want to bring to justice this animal who has obviously taken the lives of a number of people," said Nassau County Executive Ed Mangano on Monday.

Authorities would not describe what type of remains were found on Monday morning, and Smith did not detail the condition of the skull found in the afternoon.

But police have said that the bodies discovered earlier were in various stages of decomposition, including at least one that could have been there for as long as two years, according to Suffolk County Police Commissioner Richard Dormer.

The remains have included four female bodies found in December stuffed in bushes along a quarter-mile strip of beachfront property. Last week, three bodies were uncovered within a few miles of each other, fueling speculation that they could be the work of a serial killer. A woman's body was also found March 29 off Ocean Parkway, west of Cedar Beach.

The missing woman who prompted authorities to initiate the search -- 24-year-old Shannan Gilbert -- has not yet been identified.

K-9 units and dive teams had been added to the search operation in an effort to locate Gilbert, who was last seen alive in May 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.

Several neighbors also called 911, witnesses say.

Police came more than 30 minutes later, but by then Gilbert was gone and has never been heard from again.

"You want to believe everything was OK with her," Sherre Gilbert said. "But at the same time, so much time has passed it's impossible to really think that she is still alive."

Sherre Gilbert said even if her sister is never found, the search has led to one positive outcome: the discovery of the other women's bodies.

CNN's Kaj Larsen, Rupa Mikkilineni and Jen Christensen contributed to this report