Story highlights Three Brooklyn shopkeepers have been shot and killed since July

The most recent killing occurred Friday

The same gun was used in all three slayings, a police official says

The shooting death of a New York City shopkeeper Friday is raising the possibility of a three-time killer on the loose, police said.

A .22-caliber handgun is the link.

"The same gun was used in all three murders," said Deputy Commissioner Paul Browne.

But police are stopping short of calling the multiple deaths the work of a serial killer.

"It's too soon to tell," Browne said. "There are similar things in each case, but that determination hasn't been made yet."

New York Police Department sketch of the suspect

Ballistics tests on shell casings found at the crime scene indicate the .22-caliber gun used in the slaying of Vahidipour Rahmatollah, 78, at his clothing store in Brooklyn last week is the same weapon used since July in the killings of two other small business owners, also in Brooklyn.

"That doesn't say it's the same person who fired the gun, but that's the working premise, that it's the same shooter," Browne told CNN.

Rahmatollah was shot three times in the head and torso and killed inside his women's clothing store called, She-She Boutique, according to police.

Police said Sunday they wanted to talk with four "unknown individuals" seen on videotapes from security cameras in the area of the store between the time Rahmatollah was last known to be alive and the time his body was found.

"Detectives want to question them about what they may have observed because of the time of day and their proximity to the homicide scene," a police statement said.

Forensic tests show the same weapon was used in the August 3 shooting death of Isaac Kadare, 59, who was killed in his store, Amazing 99 Cent Deals. Police said he was shot in the head.

On July 6, Mohammed Gebeli, 65, was gunned down in his clothing shop, Valentino Fashion Inc. Besides being shot in the head, police said Gebeli was also stabbed in the neck.

There are distinct similarities in the way the victims were positioned after their deaths. Law enforcement sources said each victim's body was pulled away from the stores' front windows and either covered by clothes or hidden by merchandise.

"It's consistent with hiding anything from immediate view of the street, so that it's not seen from someone passing by right away," said one source. "It may have been done to give the killer more time to make (his) getaway," added another police source.

Browne said all three victims have ties to the Middle East, but for now, investigators aren't drawing any conclusions.

"There's nothing to suggest a bias," Browne told CNN. However, a task force set up to help solve the first two slayings has now been expanded to add someone from the hate crimes task force, Browne added.

"Some of the victims had an Arabic or Persian background, one was Jewish, but there was no one religion or ethnicity, although that might not be obvious to the killer," Browne said.

Some money was taken in the first two killings, raising the possibility of violent robberies. However, investigators said it's not yet clear in the most recent killing whether Rahmatollah's store had any money missing. "We're still trying to determine that," Browne said.

The FBI's website defines serial killers as "three or more killings ... having common characteristics such as to suggest the reasonable possibility that the crimes were committed by the same actor or actors."

However, Paul Callan, a former deputy chief in the Brooklyn district attorney's office who helped prosecute the "Son of Sam" case in the '70s agrees with the NYPD's hesitation to label the three shopkeepers' deaths as serial killings at this stage.

Enough isn't known, according to Callan, who is also a CNN legal analyst.

"Police will resist that characterization because they will be fearful that high publicity will alert the killer and impede the investigation," Callan said.