NEW ORLEANS (AP) — Carolyn Johnson has been sure her son is dead since he disappeared in northwest Louisiana. But more than a decade later, she says the uncertainty of not knowing what happened or where his body is never gets easier.

"You learn to live with it. But it's always there," she said in a phone interview Thursday from Sturgis, South Dakota, where she lives.

Clinton Devon Nelson was last seen about 8:30 p.m. on Sept. 1, 2006, leaving a party in the unincorporated community of Princeton. Now he's one of three missing people that Bossier Parish officials are asking the public for help in finding. The others are a woman missing for 39 years, and a man last seen 18 years ago.

Bossier Crime Stoppers put up billboards about Arrilla Webb-Vaul, Gregory Vice Jr. and Nelson on Wednesday in the parish nearly 280 miles (450 kilometers) northwest of New Orleans.

All were in their early 20s when they disappeared: Webb-Vaul in March 1979, and Vice in March 2000.

As many as 600,000 people a year are reported missing in the United States, according to the National Missing and Unidentified Persons System (NamUs) based in Fort Worth, Texas. Most are resolved fairly quickly, but there are no good national figures for cold cases, spokesman Todd Matthews said Thursday.

Lt. Bill Davis, a Bossier Parish Sheriff's Department spokesman, said it is rare for someone to intentionally disappear.

"Every blue moon you have somebody who goes missing who wants to be missing. But that's not usually the case," Davis said.

Johnson said a private donor anonymously put up a $100,000 reward Friday for information received by Dec. 31 that leads to arrest and conviction in her son's case.

Nelson had moved about six months earlier from South Dakota to work in the oilfields and be closer to his biological father, whom Johnson had divorced when he was small.

His father had taken him to a party, Johnson said. She said she learned that there had been drugs, and that Nelson, who had two broken ribs, his arm in a sling and stitches over his collarbone from a recent work injury, had a lot of money with him, and bragged about how much he was making.

"He grew up in Spearfish, South Dakota. He didn't know he couldn't trust everybody," she said.

Johnson said that the night of the party she had a horrible feeling that something was wrong but convinced herself she was overreacting. A few days later her ex-husband called, asking if she'd talked to Nelson, and saying nobody had seen him since the party. Johnson set up a Facebook page asking help finding her son.

He was 6-foot-1 (1.8 meters), about 160 pounds (72 kilograms), with blond hair and blue eyes. He wore round, wire-rimmed eyeglasses. He had faint scars above his right eye, at the corner of that eye, his right temple and jaw, from being bitten by a dog when he was 2, Johnson said.

She said Nelson's son was born about six weeks after his father disappeared and is now nearly 12.

Vice was 23 when he left his home in Princeton, about 13 miles (21 kilometers) northeast of Shreveport, to visit a friend in Taylortown, a community about 11 miles (18 kilometers) southeast of Shreveport and 15 miles (24 kilometers) south of Princeton. He left about 8 p.m. March 21, 2000, in his 1984 Chevrolet S-10 pickup truck. The truck was found three days later in Taylortown, with Vice's wallet and other personal belongings inside.

He was 5-foot-8 (1.7 meters) and 135 pounds (61 kilograms), with brown hair and hazel eyes. He had a tattoo of a white tiger on his back just behind his right shoulder.

Investigators don't think there was any connection between the two men's cases, Davis said.

Webb-Vaul had dropped her husband off at a K-Mart the evening of March 15, 1979. Her vehicle was found at the foot of the Jimmie Davis Bridge in Bossier City, with a slashed or punctured tire, and her belongings inside. Witnesses told investigators that she pulled over and a white man in a white truck immediately pulled up behind her.

She's described as 5-foot-1 (1.5 meters), 100 pounds (45 kilograms), with blonde hair, blue-grey eyes, and a small mole on the right side of her nose.

"You think, 'Gee, nobody's going to know something that far back,'" Davis said. But he said the billboards may spark someone's memory.

Johnson said she's not out for vengeance or even justice.

"I just simply want to bring my son home. And I want to give him some form of a resting place and be able to tell his son that we know where he is," she said.