But a tall, blue electronic sentinel stands just around the bend — and sources say it puts prime suspect Darryl Littlejohn at the scene about 2 1/2 hours before St. Guillen's corpse was found.

The NYPD traced Littlejohn to that lonely corner of East New York, off the Belt Parkway, by tracing the invisible "pings" that his T-Mobile cell phone sent to the antenna-studded tower, sources said.

The big, blue tower apparently took notice that Littlejohn's cell phone was nearby, even though he wasn't making a call — and it stored that information, which was later retrieved from T-Mobile by cops.

"It's a way to track people that is stronger than relying on witnesses," a police source said.

The Daily News found the tower, about four blocks from where St. Guillen was discovered, and at least a dozen more along the 5.7-mile stretch between the site and Littlejohn's Queens home.

On Sunday, as he revealed blood evidence allegedly tying Littlejohn to the slaying, Police Commissioner Raymond Kelly said cell phone records helped nail down the suspect's whereabouts.

But he was careful not to say they were records of actual calls.

"There is telephone evidence, telephone records, that put the telephone that Mr. Littlejohn had in his possession in the vicinity, the immediate vicinity, of where the body was located — and also the route to that location," Kelly said.

Cell phone companies and industry groups either told The News they didn't save location data or didn't respond.

But experts say few rules govern that information.

"The phone company can have records of where you are whenever your phone is turned on. It's up to the phone company how long they store those records," said privacy expert Peter Swire, a law professor at Ohio State University.

"Most people don't realize this. And criminals don't think to turn their phone off when they're about to commit a crime."

St. Guillen, 24, vanished early in the morning of Feb. 25 after a night of drinking that ended at The Falls bar in SoHo.

Littlejohn, a 41-year-old ex-con, was working as a bouncer and was seen outside the bar with St. Guillen, witnesses said — within sight of a white cell phone antenna atop a brick building on the corner of Lafayette St.

St. Guillen's bound and tortured body was found about 8:40 that night after cops were tipped by an anonymous 911 call from a public phone at a Linden Blvd. diner — where more cell phone antennas stand watch from an apartment complex nearby.

Police say Littlejohn is their prime suspect and searched his small yellow house in Jamaica — where a nest of cell phone antennas are clustered atop a red brick apartment building just a few blocks away.

Cops say they have traced at least part of Littlejohn's route that day through studying his cell phone "pings" — leaving his home about 5 p.m., for example, and reaching the body-dumping site an hour later.

Police and prosecutors commonly subpoena cell phone data to see suspects' call records.

And a federal rule requires all cell phones to be able to pinpoint their locations within 1,000 feet when 911 is dialed.

Cops were alerted to St. Guillen's body by an anonymous call to 911.

Littlejohn's lawyer Kevin O'Donnell said flatly on MSNBC's "Rita Cosby: Live and Direct" last night that his client was "not the one that called 911."

He also discounted a witness claim that Littlejohn drove away in a blue van with St. Guillen. Littlejohn owns such a van, but O'Donnell said it has been in his client's driveway for the past three months because "it is completely inoperable."

Privacy experts say few people realize cell phone antennas can track their whereabouts even if they don't make a call — and that cops can tap into those records.

"The concern is that we are all carrying tracking devices around with us all day, so that every place we visit is logged," said Electronic Frontier Foundation attorney Kevin Bankston, who has pushed for strict privacy controls on cell phone data.

"You are creating a detailed map of all your comings and goings," he added. "Information about your past locations can come back to haunt you."

How signal locates you

When your cell phone is switched on, your network provider knows exactly where you are within 300 YARDS or less.

When you make a call with your cell phone, its message is picked up by one of the aerials operated by service providers.

But even when you are not making a call, your phone is sending out a regular "check" signal. In major cities you will be within range of several phone masts. Computers can compare the signal strength emitted by your phone and the tiny time lags at each aerial, giving the operator several accurate readings. By intersecting the readings from at least three masts, it can be calculated where your cell phone is at any given time.

Originally published on March 15, 2006