A nationwide FBI bulletin could not be blunter: "Harrington reportedly is suicidal, has violent tendencies and serious mental health issues," it says.
The bulletin mentions reports that Harrington and the children's mother had been on rocky terms recently. After his abrupt disappearance, police say they examined letters left by Harrington that suggest he is planning to kill the children.
"I am asking the public to please help me," the children's mother, Agena Battle, tearfully pleaded at a Wednesday press conference in Columbus, Ga. "I want my children home where they belong with me."
"My daughters, Aliyah and Agena, are only 23 months old. My son, Cedrick, he is 3 years old, he's a 3-year-old boy. My children cannot take care of themselves. Please help me."
"Please help me, and keep them in your prayers."
Battle said she called Harrington's parents, who had not heard from him, before she called police.
The FBI believes Harrington might have ties to Tennessee. Authorities in that state received several tips after the neighboring state of Georgia issued an Amber Alert. But the stories conflicted, and sightings of the kids at a motel and the car's heading north on Interstate 45 were not confirmed, said Kristin Helm, a spokeswoman for the Tennessee Bureau of Investigation.
"We have kept the Amber [Alert] active on our Web site, but it hasn't generated any new leads," Helm told ABC News.
FBI special agent Stephen Emmett told ABC News that with each day that goes by the situation gets more perilous for the children.
Harrington is thought to be driving a green 2002 Chevrolet Impala with Indiana license plate 93-L4740.
According to the Justice Department and missing children advocacy groups, the children in this case are among the roughly 69,000 kidnapped each year. The overwhelming majority, 82 percent, are abducted by family members.
Georgia issued an arrest warrant for Harrington for cruelty to children on March 6. Four days later, a judge in the federal court for the middle district of Georgia issued an additional arrest warrant for unlawful flight to avoid prosecution.