Daniel Ruiz was 20 when he was shot and killed in Dallas on June 28, 2016. He had arranged to sell a cellphone through the Offer Up app and went to meet the prospective buyers in a public place accompanied by two friends, one of whom was 12. Four men arrived and attempted to rob Daniel of his car; when he refused to hand over the keys one man produced a gun and shot him in the chest. He was taken to a local hospital but died of his injuries.

The suspects fled in a white getaway car and the case remains unsolved. Dallas police have urged anyone with information to come forward; Crime Stoppers is offering a reward and the department circulated a bait ad and has discussed creating exchange zones at patrol stations after linking Daniel’s slaying to two other Offer Up–related robberies in the city. The family has spoken publicly about their grief and desire for justice.

If anyone knew Daniel or saw the incident or the white getaway car, can you share what you know with investigators?