D.C. police officer Richmond Phillips fatally shot his former mistress and then let their young daughter "cook" in a car that had its windows rolled up on a hot spring day, a prosecutor said.

Phillips' trial in the deaths of 20-year-old Wynetta Wright, and Jaylin Wright, 11 months, started Monday in Prince George's County Circuit Court. He is charged with two counts of first-degree murder, one count of child abuse resulting in death and a handgun charge. If convicted, he could be sentenced to life in prison without the possibility of parole.

During opening arguments, Prince George's County State's Attorney Angela Alsobrooks said that Phillips killed his former lover and their daughter because he did not want to pay child support.

Phillips and Wynetta Wright had engaged in a sexual relationship for several months, but the married police officer wanted nothing to do with his mistress when she told him that she was pregnant, Alsobrooks said. After Jaylin's birth, Wynetta sought child support from Phillips, and a paternity hearing was scheduled.

The night before the May 2011 hearing, Wynetta, with her daughter in her car, met Phillips in a park near Prince George's County's border with the District. Phillips allegedly put a gun to Wynetta's head, shot her and dragged her body to a wooded area. He then drove Wynetta's car a short distance away and left Jaylin in the vehicle.

"That baby cooked," Alsobrooks said, noting that there was a heat advisory on the day of the child's death.

Phillips' lawyer, Brian Denton said during his opening argument that his client did not commit the murders.

"He didn't do this thing," Denton said, adding that the evidence was circumstantial.

The trial is expected to last for about six days.