DALLAS — Dallas police were called to investigate two separate double homicides within an hour late Thursday and early Friday.

Demetris Tillis, 24, and an undentified 15-year-old girl believed to be Tills' girlfriend were found dead in an apartment at 9601 Forest Lane in northeast Dallas just before midnight Thursday, Dallas police Sgt. Gil Cerda said.

Investigators said the victims had been robbed, but a motive for the killings has not been determined and no suspects have been identified. Police said the couple died violently but had not yet determined how they were killed.

An hour later, police were called to the 2800 block of Cleveland Street, south of downtown Dallas, where they found Rodrick Green and Charles Cancel, both 24, shot to death.

The men's bodies were found dead at a known drug house, Sgt. Cerda said.