“His murderer should hear the screams of his mother to understand what he did today,” the Nassau County executive, Edward P. Mangano, said of the fallen officer and the gunman, pausing to take breaths at the news conference. “We can best honor him by hunting down this murderer.”

Police helicopters hovered above the area as night began to fall; officers canvassed the known addresses of a man believed to be the suspect. Around 6:35 p.m., the Nassau officials identified the suspect as Darrell Fuller, a Queens resident who was on parole after serving less than five years for attempted murder and who was arrested in 2010 on charges that he sold crack cocaine; a $50,000 reward for information leading to his capture was also announced.

The arrest came shortly after. A 911 call came in reporting a man shot on 111th Avenue in St. Albans about 6:45 p.m.; officers found Mr. Fuller in a minivan with a gunshot wound in the torso. The police said the wound appeared to be self-inflicted. Mr. Fuller, 33, was arrested and taken to Jamaica Hospital Center, where he was in guarded condition late Tuesday.

Before the arrest, at the news conference in Nassau County Police Headquarters in Mineola, James Carver, the president of the Nassau Patrolmen’s Benevolent Association, called for stricter punishments for killers of the police and said Officer Lopez’s death had rocked the force, coming less than a week after another officer, Joseph Olivieri, a highway patrolman, was killed when he was hit by a car while responding to a collision. Officer Lopez attended the funeral.

“To say that this is a difficult time would be an understatement,” Mr. Carver said, “We are still reeling from the loss of our brother from five days ago.”

Officer Lopez, 29, was honored in 2010 by officials from the Town of Hempstead for his actions in saving a man’s life. He was also a volunteer firefighter with the Dix Hills Fire Department and an emergency medical technician.