ALBANY, New York (CNN) -- At the age of 21, Christopher Jenkins appeared to have everything going for him. The University of Minnesota senior was good-looking, had a near perfect grade-point average and had a future in business.

Christopher Jenkins, 21, vanished on Halloween 2002. Four years later, police ruled his death a homicide.

Then, suddenly, he vanished.

He was last seen celebrating Halloween at a bar in downtown Minneapolis, Minnesota, in 2002. Jenkins' friends said he left about midnight. Four months later, his body was found in the Mississippi River, still wearing his Halloween costume.

Minneapolis police classified the drowning as accidental.

Jenkins' blood-alcohol level was well above the legal limit, and police told his parents that he'd probably had too much to drink after bar -hopping with friends. They thought he'd fallen into the river.

Despite a lack of evidence, his parents, Steve and Jan Jenkins, insisted that there had been foul play.

"He was loaded into a vehicle, a van, driven around and eventually murdered," Jan Jenkins told CNN. "He was murdered and thrown away like a piece of trash."

Hundreds of miles away, Kevin Gannon, a retired detective with the New York Police Department, was investigating the mysterious deaths of several college men from New York state. Each of the deaths had been ruled an accidental drowning. Watch how clusters of drowning deaths raise suspicion »

In 2006, nearly four years after Jenkins died, there was a break in the case. A tip from a man in jail, described by Minneapolis police as a witness or suspect, caused police to change Jenkins' cause of death from "unexplained drowning" to homicide.

It was a lucky break for Gannon. He had promised the parents of Patrick McNeill that he wouldn't quit until he'd found out how the Fordham University student died. McNeill's body washed up in the East River two months after he left a bar in New York.

Gannon enlisted the help of another former NYPD officer, Anthony Duarte, when Christopher Jenkins' death became a homicide. In 2003, the two traveled to Minneapolis to investigate Jenkins' death.

They learned about a string of student drowning deaths, many of them involving young men who attended colleges along the Interstate 94 corridor in the Midwest -- in Minnesota, Wisconsin and Iowa.

Nine of the deceased attended the University of LaCrosse, in Wisconsin. Three attended colleges in New York state.

In all, the investigators say they've connected the bizarre drowning deaths of at least 40 college-age men across the country.

The two detectives believe that in each case, and in others they investigated, the men were drugged and then their bodies were slipped or tossed into the water to make it appear as if they'd drowned.

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Why would the killer or killers put the bodies in the water? The effect of water on evidence makes for an almost perfect crime, Duarte said. Not only does it make it appear like an accidental drowning instead of a murder, but the water frequently washes away key pieces of evidence such as fingerprints and fibers, so the killer can't be identified.

Together, the two detectives began mapping out the drowning deaths and working the case backward. Instead of focusing on where the bodies had been found, they used GPS devices and tracked river flow patterns and water levels to figure out where the bodies entered the water.

As Gannon and Duarte investigated the deaths, they began to see a trend. The cases spanned 25 cities in 11 states, and at least some of them were connected by a creepy symbol left near the water's edge: a smiley face painted on trees and other surfaces.

The detectives believe that the smiley faces were left by the killer or killers. They varied in size, with each face more haunting than the next.

The most sinister was found in Iowa. It was drawn in red with a devil's horns. Next to the smiley face was a note that read, "Evil Happy Smiley Face Man."

Asked whether he believed there was a hidden message in the smiley faces, Duarte told CNN, "The message is, they're taunting the police."

Duarte and Gannon said they found 12 other matching symbols similar to gang graffiti. But, to protect their investigation, they wouldn't describe them in detail. The detectives say the string of deaths could be the work of more than one killer because some of them took place on the same day in different states.

"It's so widespread. We have so many different victims in so many different areas," Duarte said. "It would, in my view, be impossible to be one person."

The detectives also believe the victims were targeted. All of the young men were popular, athletic and good students.

Who would commit this type of crime?

"The type of person that would be the opposite, not smart, someone not good in school, maybe doesn't have a job, not popular," Duarte said.

Gannon and Duarte believe that the young men were drugged to weaken them and given a substance that couldn't be detected by an autopsy.

"I believe these young men are being abducted by individuals in the bars, taken out, at some point held for a period of time before they're entered into the water," Gannon said.

He also believes the victims were abused mentally and sometimes physically before they were killed.

"This is a chance for them to have power and control over somebody else and manipulate. The fear of death is just as important as the act of death itself," Gannon said.

Minneapolis police are not convinced that Jenkins' death was the work of a serial killer.

"Although we have collaborated with investigators from the FBI and communicated with other jurisdictions in which similar drowning deaths have occurred, we can neither confirm nor endorse the 'smiley face murders' theory currently being publicized," the department said.

The FBI also has its doubts.

"To date, we have not developed any evidence to support links between these tragic deaths or any evidence substantiating the theory that these deaths are the work of a serial killer or killers. The vast majority of these instances appear to be alcohol-related drowning," the bureau said in a statement.

Still, Gannon and Duarte are concerned that the killing could continue. They say whoever killed Jenkins might already be stalking other college students.

"Unless you've been out there to the scenes to evaluate [them] yourself, if you haven't done that, you're basically Monday-morning quarterbacking," Gannon argued.

Duarte thinks local police forces did not investigate the cases adequately and that the FBI could have done more, too.

"I don't think any of them went out to the field and beat the bushes," he said.

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