DENVER, Colo. - At first it seemed like they had simply vanished. Three Denver area women, all young, all with an independent streak. At first their families thought little of it, until the evidence started to mount that something was seriously wrong.

Bob and Noelle Marcum, living in Springfield Illinois began to worry about their daughter Jennifer in 2003, when she didn't return their calls. Then one day, almost a year later, the FBI called, saying something bad had happened.

The Marcums kept asking questions, until the FBI set up a meeting with a mysterious man, who supposedly knew what happened to Jennifer. His name was Scott Kimball. And, after meeting him, the Marcums decided he was the one who had killed their daughter.

In Wheat Ridge Colorado, Rob McLeod was worried because his daughter Kaysi ran away. Kaysi was living with her mother Lori McLeod, Rob's ex-wife. When Kaysi never came back home, Rob McLeod had friends run a background check on everyone in Kaysi's life. He got a hit -- Lori McLeod's new husband had a lengthy criminal history, his name was Scott Kimball.

In Centennial, Howard Emery was worried because his daughter, LeAnne went missing on a caving trip to Mexico. LeAnne had a boyfriend in prison, who was sure he knew what happened: LeAnne was mixed up with a dangerous man who went by Hannibal. His real name was Scott Kimball.

In 2006, the Marcums were featured in an article in the Denver paper Westword, and mentioned the name Scott Kimball. Rob McLeod read the article and realized the cases were all linked.

The two dads took the information to the FBI thinking the case was a slam dunk. What they didn't know was the FBI was already very familiar with Scott Kimball --for years Kimball had been a paid FBI informant, according to Lori McLeod's, Kimball's ex-wife.

Everytime Kimball got in trouble, he seemed to get out by turning informant on another prisoner, and convincing authorities he needed to be released. The feds actually let Kimball out of jail to be an informant on a suspected drug ring leader Steve Ennis. Kimball linked up with Ennis' girlfriend, Jennifer Marcum, and that's when Marcum disappeared.

According to court documents, the FBI thought Marcum had probably been killed. But they never tied the death to Kimball. At the same time, Howard Emery was trying to lead the FBI to Scott Kimball in his own daughter's disappearance. But investigators didn't follow up on his leads.

Even when Kimball's stepdaughter, Kaysi McLeod, and Kimball's uncle Terry Kimball disappeared, the FBI didn't connect the dots. It took Bob Marcum and Rob McLeod to convince investigators to focus on Kimball.

This fall, Kimball plead guilty to two counts of Second Degree Murder, and was sentenced to 70 years in prison. But the families of his victims wonder if the killing spree could have been stopped, if the FBI would have focused on the actions of their own informant.

The victims' families are pleased the FBI eventually helped convict Kimball, and helped recover three of the four victim's bodies. But Kimball has never given up the exact location of Jennifer Marcum's remains. And a source close to the investigation tells FOX 31, the FBI is now looking at whether Kimball killed a fifth victim in Alaska.