A police officer and a police dog search for Travis Boys, who is suspected of killing New Orleans police officer Daryle Holloway, in New Orleans, United States, June 20, 2015. [Photo/Agencies]

Department spokesman Tyler Gamble said police were trying to determine what weapon Boys used and how he obtained it, but do not believe Boys used the officer's gun.

A helicopter circled overhead as marked and unmarked units from state police and other law enforcement agencies cruised the side streets searching for the suspect.

Harrison said Holloway was not the arresting officer but was transporting Boys to a jail when the shooting occurred.

Holloway had been a member of the New Orleans Police Department since 1992. He was the father of three children.

Harrison said he met with two of those children and Holloway's former wife at the hospital after he died. "As a new chief, it was the hardest thing I've ever had to do in my life," said Harrison, who became chief last year.

Mayor Mitch Landrieu decried the killing as "the lowest of the low" and called on the public to help police with information on Boys' whereabouts.

"Killing an officer in the line of duty is an attack on our community that will not stand," Landrieu said in a statement. "The heart and soul of New Orleans is heavy today as our community mourns one of our city's finest."