By Bill Fuller

Associated Press

SHREVEPORT, La. — A man who was already wanted in a second-degree murder case is now the key suspect in the shooting death of a Louisiana police officer, authorities said Thursday, as they vowed to "scour the Earth" to find him.

Authorities launched a wide manhunt for the suspect, 27-year-old Grover Cannon, who was wanted in connection with another murder case when he allegedly shot and killed Police Officer Thomas LaValley on Wednesday night.

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"We will scour the Earth. No matter where he goes we will find him," Shreveport Police Chief Willie Shaw said at a news conference. Police spokesman Bill Goodin says Shreveport police, Caddo Parish deputies, along with DeSoto Parish deputies, U.S. Marshals and the state police have joined the search for Cannon.

LaValley, 29, was shot multiple times while answering a call about a suspicious person at a home in the city's Queensborough neighborhood. Shaw said neighbors told the officer that the man inside the home was wanted. Shaw says the officer didn't know the man was armed, and he went inside and was shot. He was transported to University Health.

Police already had a warrant for Cannon's arrest on a charge of second-degree murder, which carries an automatic life sentence; they now have one out for him for first-degree murder, which carries a possible death penalty.

Asked if Crime Stoppers had set a reward, Shaw said, "Not at this point. We don't need that incentive. We will find him. He is our No. 1 priority to locate."

"No matter what rock he crawls under we will find him," Shaw said. "I would suggest that he turn himself in immediately. Because we are looking for him. I say 'we' in the biggest sense of the word. We are not bound by state lines. We are not bound by county lines. We are not bound by country lines. ... We will not rest until this individual is brought to justice."

A four-year veteran of the force, LaValley began working as a lawman after more than three years as a television news photographer. Shaw described Lavalley as "one of our top young people" and a hero who will never be forgotten.

"He was doing what he loved," Shaw said. "He did not hesitate to confront a bad person."

Shaw said LaValley came to his office while working at the television station and said he'd applied for a job but had not been hired.

"I told him to apply again, and if he qualified, he would be hired."

LaValley didn't believe him, Shaw said, but he assured LaValley that he would be hired if he met the requirements.

The mayor of Shreveport issued a statement decrying the death.

"Last night, we lost one of our brave, uniformed officers in the line of duty. Our hearts are saddened," Mayor Ollie Tyler. "We ask for the community's prayers for this officer's family and SPD as we grieve the loss of one of our own who paid the ultimate sacrifice while serving and protecting the citizens of this city."

Copyright 2015 The Associated Press