Police in Ohio are searching for a man who fatally shot a “random” victim and posted the footage on Facebook.
The suspect, Steve Stephens, later said in a separate video post that he had killed 13 people and would kill more.
Cleveland police confirmed one killing but said on Monday morning that they did not know of any other victims.
Mr Williams said that “multiple forces” were looking for Mr Stephens, who “needs to turn himself in”.
The victim has been identified by Cleveland police as 74-year-old Robert Godwin.
The father of nine was shot and killed in a residential area of the city while walking home from Easter lunch, according to WOIO-TV.
City police chief Calvin Williams said in a Monday morning press conference: “Steve is still out there some place… we’re still asking Steve to turn himself in but if doesn’t we’ll find him.”
In the graphic video the suspect approaches Mr Godwin and asks him to say the name of a woman believed to be Mr Stephen’s former girlfriend, before shooting the elderly man in the head.
The woman, Joy Lane, confirmed to CBS News that they had been in a “relationship for several years”.
“I am sorry that all of this has happened. My heart & prayers goes out to the family members of the victim(s). Steve really is a nice guy… he is generous with everyone he knows.
“He was kind and loving to me and my children,” she wrote in a text message.
Maggie Green, the suspect’s mother, told CNN that he was “mad with his girlfriend. That’s why he is shooting people and he won’t stop until his mother or girlfriend tell him to stop.”
Police have put out alerts to local authorities in Ohio, New York, Indiana, Michigan, and Pennsylvania over the “senseless” incident, and urged people not to approach the suspect, who he said was likely to remain armed and dangerous.
The Cleveland police department issued a photo of Mr Stephens, 37, on its website, describing him as a 6ft 1in (1.9m) tall black male of medium complexion.
On Monday morning FBI Special Agent Stephen Anthony said they are “doing everything humanly possible” to find the suspect, but he added, “quite frankly, he could be in a lot of places”.
US Marshall Peter Elliot added that law enforcement agencies across the US will be assisting in the investigation, and will “make this individual’s world very, very, very small”.
He is thought to be driving a white Ford Fusion, but may have switched cars.
Police say the victim appeared to have been selected at random in what he described as a “senseless” murder.
He added that Mr Stephens “clearly has a problem” and threatened to prosecute anyone who is aiding him.
The Federal Bureau of Investigation (FBI) is helping police investigate the incident.
The mayor of Cleveland, Frank Jackson, has spoken directly to the victim’s family to express his condolences.
Mr Stephens is employed by Beech Brook, a behavioural health agency that works primarily with children and families, according to a company spokesman.
Facebook said in a statement on Sunday that it co-operates fully with the authorities in incidents where there are “direct threats to physical safety”.
Responding to the reports involving Mr Stephens, the company said that it was “a horrific crime”, adding that it “does not allow this kind of content” on its site.