Police in Cleveland, Ohio said they believe the suspect, 37-year-old Steve Stephens, is armed and dangerous.
Federal authorities offered a $50,000 reward for information leading to Stephens’s arrest.
Cleveland police on Monday alerted authorities in four nearby states — Pennsylvania, New York, Indiana and Michigan — to be on the lookout for Stephens. Later in the day, they expanded their search nationwide.
“Our reach now is basically all over this country,” said Cleveland police chief Calvin Williams.
“This is what we would consider a national search.”
WATCH: Police urge Facebook killer to surrender
Robert Godwin Sr, 74, was shot and killed on Sunday, apparently at random, as he walked home after an Easter dinner, police and local media reported.
“We know that Steve is still out there someplace,” Williams said. “We don’t know his condition and, of course, right now we don’t know his location. We’re asking the public to remain vigilant.”
Police issued an arrest warrant for aggravated homicide against Stephens, described as a six-foot one-inch, 244-pound (1.85 m, 110-kilo) black man with a full beard who was last seen in a white Ford Fusion with temporary license plates.
Authorities said that they were assuming he was still with his vehicle more than 24 hours after the murder.
In a video uploaded to Stephens’s Facebook page, a man resembling him is seen exiting a vehicle and approaching his victim, who is shown with a gun to his head and then falling to the ground after a shot is fired.
“I killed 13, so I’m working on 14 as we speak,” the suspect says in a second video clip. “I’m just driving around hitting motherfuckers, man. I just snapped man, fuck.”
So far, however, police say they are aware of just one victim killed by Stephens.
‘Mad with his girlfriend’
Since the shooting, Facebook has taken down Stephens’s videos.
“This is a horrific crime and we do not allow this kind of content on Facebook,” the company said in a statement.
“We work hard to keep a safe environment on Facebook, and are in touch with law enforcement in emergencies when there are direct threats to physical safety.”
Stephens’s mother told CNN she called him on Sunday after learning about the video and he told her he was shooting people because he was “mad with his girlfriend.”
Police said the woman he referred to was in a safe place.
Cleveland detectives also made contact with Stephens by phone early in the investigation, Williams said.
“They tried to, of course, convince him to turn himself in and, of course, that hasn’t happened to date,” he said.
Earlier reports that Stephens might be on the run in Pennsylvania were not correct, police said.
Dozens of searches
The Federal Bureau of Investigation and the US Marshals Service have joined the hunt.
“This man is dangerous and he could be nearby, he could be far away, anywhere in between,” said Stephen Anthony, the FBI special agent in charge.
“We’re going to make this individual’s world very, very, very small, so I’m very optimistic,” said Peter Elliot, the marshal on the case. “I think things are going to happen, and I believe things are going to happen pretty quickly.”
But Williams said police had searched dozens of locations overnight “to no avail.”
“If there’s somebody who is helping Steve or think you’re helping Steve, you’re really not. You’re going to get yourself in trouble, along with him,” he warned.
Stephens worked for Beech Brook, a behavioral health agency serving children through mental health services, foster care and adoption, at-risk youth and other programs.
In his video, Stephens displayed his Beech Brook badge.
“We are shocked and horrified like everyone else,” Nancy Kortemeyer, a spokeswoman for the facility told CNN.
“To think that one of our employees could do this is awful.”