EL CAJON, Calif. -- Authorities are looking for a witness who may have been in the parking lot of an East County liquor store where clerks were gunned down during a robbery last week.

Detectives received information that a witness was in a yellow car, possibly a Nissan or Mitsubishi, shortly after the murders of Heather Mattia, 22, and Firas Eiso, 23, at Granada Liquor in El Cajon on March 1.

Detectives are asking the witness to contact authorities in order to help catch the bandits who killed Mattia and Eiso, police said.

A $100,000 reward is being offered by authorities in conjunction with the California Independent Grocers Association for information leading to the arrest and conviction of the robbers.

Footage from the business's security cameras show the perpetrators walking through a front door, then emptying a cash register.

At some point during the heist, Eiso and Mattia were shot in the back of the heads. A friend found their bodies during a visit to the store shortly before midnight.

The bandits' faces were largely obscured by hooded sweat shirts, one of which had the words "New York" across the front.

Authorities have little in the way of descriptions to work with, police said.

Anyone with information in the case was urged to call the San Diego County Crime Stoppers anonymous tip line at (888) 580-8477.