San Jose police have turned to the public for help in solving four murders that bear some similarity but no known connection to each other.

The killings, which took place over a four-year span, all involve victims of Vietnamese descent who were shot point-blank at cafes known to draw Vietnamese gang apparatchiks. To generate new leads in the cold cases, the San Jose Police Department recently released new details and graphic surveillance videos of the assassinations.

The first homicide took place just before 3am on April 6, 2013, outside of the now-shuttered Elite Car and Audio Performance on Senter Road in south San Jose. Police responding to a 9-1-1 call found 37-year-old gunshot victim Viet Tran bleeding to death in the parking lot. Investigators determined that a fight erupted at a party inside the business before spilling outside.

Some attendees barricaded themselves in the car audio shop, prompting police to use less-than-lethal projectiles to force them out. Officers detained and questioned 25 people that night, which confirmed a link to several Vietnamese street gangs but failed to identify a motive or a suspect.

Police also shared surveillance footage of a shooting on Dec. 5, 2014, that took the life of a 41-year-old man named Thach Duong. Grainy video shows two men walking right up to the victim outside the Golden King Café on Alum Rock Avenue in East San Jose and firing off several rounds.

The gunman, his face partially obscured by a black hoody, looks thin. A man who accompanies him wears a baggy jacket and beanie. It’s hard to make out their faces.

Another slaying took place on March 11 this year at the Bon Mua Café on Tully Road. Officers found two gunshot victims, one survived and the other—43-year-old Brian Le—died in the hospital a short time later.

Footage from a security camera shows the unidentified gunman in a beanie and baggy sweat suit walk in through the back door and jog out a short time later.

In an interview with NBC Bay Area, Lt. Paul Spagnoli called the shooting a “very deliberate act, very quick.” The speed in which it was executed made it tough to track down a suspect.

The most recent homicide took the life of 26-year-old Justin Tran. Police say the gunman pulled up in a black Lexus sedan, parked it in front of Tran’s car, walked up to his driver-side window and shot him at point-blank range.

Police urge anyone with information about the homicides to call detectives at 408.277.5283. Those who wish to remain anonymous can call Silicon Valley Crime Stoppers at 408.947.7867.