Satellite imagery. Laying bare the thick brush of the burial site. Relentless interviews with taxi drivers and others who might know something about a victim's last day.
These are among the investigative techniques that police have employed in serial killer investigations like the increasingly complex probe facing Suffolk cops along Ocean Parkway.
With the recent shocking discovery of four more bodies just off the roadway from Gilgo to Oak Beach, police may have stumbled upon a scene rich with physical evidence and other clues. At the same time, the additional bodies have taken the probe to a new and difficult phase, raising the pressure on police to find the serial killer or killers, law enforcement experts said.
Among the questions for police now, said Joseph Pollini, a professor at John Jay College of Criminal Justice and a former NYPD homicide investigator: "Why did this go on for so long? How did eight bodies wind up in the same general area? It's not their [the police's] fault, of course, but the public is going to see that and say, 'How did the police not notice?' "
Suffolk police say they are throwing the full extent of their resources into the case, and the search for bodies will extend into Nassau County starting Monday. Experts say it's clear police have been pursuing the case aggressively.
"They're doing everything right," said Louis B. Schlesinger, also a John Jay professor. "These are very difficult investigations . . . It could go on for 30 years and sometimes it's never resolved."
Experts also warn that, while they can't predict what will come next, discovering another set of bodies along Ocean Parkway is not beyond the realm of possibility.
"My concern is that there could be another site," said Vernon Geberth, former head of the NYPD Bronx homicide squad. "If you have a cluster of four and another cluster of four, you may have another cluster of four. This guy appears to be quite a prolific serial killer."
No one has connected the first set of bodies with the next four, however. And so far, police say they don't even yet know the gender of the four that were found in the past two weeks. The first four, found in December, have all been identified as young women who went missing after advertising as escorts on Craigslist.
Geberth said in cases where bodies are dumped in deep brush, police elsewhere have brought in an elite tactical tracking unit of experts to find clues left behind by killers and the victims' bodies. Once an area has been scoured, police in some cases have even deforested an entire area of underbrush to give investigators a clear view of what's there.
Geberth said this was done in the probe of the murder of 10-year-old Walter Contreras Valenzuela in Morris County, N.J., in 2001. After finding the boy's body, police cleared the area. They then found the weapon used to kill him, which also had been tossed into thick brush in the search area. That in turn led to the killer, said Geberth.
A key to any successful murder investigation is publicizing numbers, such as 1-800- 220-TIPS, and some experts urged a greater effort in that direction.
"A lot of times these cases are not solved on great detective work, believe me," said Jay Salpeter, a former NYPD homicide detective and now a private investigator based in Great Neck. "One of those phone calls could be the one that breaks it."
Suffolk police declined to comment on the experts' suggestions.
Geberth said that in their hunt, police elsewhere have used a little-known technology -- past NASA satellite images to see if there were any vehicles in the area where a victim was last seen.
Geberth said satellite images were used in the case of "The Green River Killer" in the Pacific Northwest.
NASA officials had no immediate comment.
Of course, a major source of clues in Suffolk will come from identifying the four new bodies and developing full profiles of who they were and who they had contact with. Geberth said weeding through their cell phone and email records will be essential. "There has to be a common denominator" among the victims, he said. "There has to be an electronic trail."
One Suffolk County investigator, who declined to be identified because of the ongoing investigation, told The Associated Press that detectives are poring over credit card records of the victims to track their movements and determine whether they spent money in the area.
Cell phone records are key in tracing their movements. In the days after one victim, Melissa Barthelmy, went missing on July 12, 2009, a man made six calls from her cell phone to her teenage sister, a law enforcement source told Newsday in January.
New York City police monitoring the phone determined the calls were coming from midtown Manhattan, but the caller didn't stay on the line long enough for police to find him, the source said.
Joe Coffey, a retired NYPD detective sergeant who worked on the Son of Sam case, said police may be interviewing taxi drivers, since some of the women may have relied on them.
Many experts agree on one aspect of the case: The killer or killers are most likely locals. "He's a local guy. He's in that general proximity somewhere because he's very comfortable in that area," Salpeter said.
Sexual serial killers often revisit the "disposal site" where they left the bodies so they can "relive" the moment, Geberth said. "This is abnormal psychology at its most extreme," he said.
"These are very, very rare events," Schlesinger said. But Suffolk and Nassau police are "going to do whatever they feel is necessary. There is nobody that wants this case solved more than law enforcement."
With Sophia Chang