INDEPENDENCE, Mo. -- Independence police called off an Amber Alert they issued Saturday night after they said a mother abducted her four children -- two boys and two girls.

The children are in state custody, and the state Division of Family Services gave the mother an unsupervised overnight visit with them.

She was to return the children at 2 p.m. Saturday but she did not, according to a news release from the Independence Police Department.

The mother is Shirley A. Riggs, 39, according to police. She is 5 feet 5 inches tall and weighs 115 pounds. She has brown hair and brown eyes.

She may be driving a maroon 1992 Dodge Caravan minivan with Missouri license MB6-C8M, according to police.

A warrant was issued for her arrest Saturday with bond set at $500,000.

The children are Kelley L. Riggs, Raven S.A. Riggs, Rhiannon G. Riggs and Spencer D. Riggs, according to police.

Kelley Riggs, a boy, is white, 10 years old, 5 feet 1 inch tall, weighs 67 pounds and has brown hair and blue eyes, according to police.

Raven Riggs, a girl, is white, 12 years old, 5 feet 1 inch tall, weighs 67 pounds and has brown hair and brown eyes, according to police.

Rhiannon Riggs, a girl, is white, 7 years old, 4 feet 2 inches tall, weighs 45 pounds and has brown hair and brown eyes, according to police.

Spencer Riggs, a boy, is white, 14 years old, 6 feet 2 inches tall, weighs 180 pounds and has brown hair and brown eyes, according to police.

They may be headed to Denver or Oklahoma, according to Independence police. They called off the Amber Alert Monday because they didn't believe they were still in the Kansas City metropolitan area, they said.

It's the fourth time she's taken the children out of the state, according to police, and she's previously taken to them to New Mexico and Oregon.

The mother was scheduled to have a custody hearing on Monday and, according to police, is afraid of losing custody of her children. Independence police said they fear she may be unstable and desperate and that the children might be in danger.

Lois Hinman, Shirley Riggs' mother, said, "Her children are her world. That's it right there, and she got scared, panicked and ran because she thought she was going to lose them."

Hinman said he daughter lost custody of her children because of a complicated legal loophole during divorce proceedings from her husband.

Due to her frustrations, she took her children to Oregon and New Mexico without the court's permission two other times, Hinman said, and she served six months in prison as a result.

"I'm worried. I'm worried about it all, what will happen next and where she's at and are they eating," Hinman said. "I'm sure she'll manage."

Authorities urge anyone who sees them or has any information about their whereabouts to call police or call the Crime Stoppers TIPS Hot Line at 816-474-TIPS (8477), text TIP452 plus a message to CRIMES (274637) or submit a tip online at kccrimestoppers.com.