Colorado has become a major exporter of illegally grown marijuana to the rest of the country, according to a new report by a network of law enforcement organizations.

Last year, police across the country made at least 274 highway seizures of marijuana that investigators linked back to Colorado. According to the report, the seized pot — 3½ tons of it in 2012 — was destined for 37 different states, most frequently Kansas, Missouri and Illinois.

Many of the cases involved multi-pound quantities of marijuana being shipped out of state. Officers also seized hundreds of thousands of dollars connected to the cases, the report states. Some of the seized marijuana was diverted from medical-marijuana dispensaries, the report alleges.

In addition, U.S. Postal Service inspectors last year seized 158 packages of marijuana being sent through the mail, according to the report. They seized 209 packages of pot in the first five months of this year alone.

All of the reported seizure numbers are significantly higher than they were several years ago. In 2005, for instance, police made 54 highway seizures of Colorado-grown marijuana, according to the report.

The report was written by the Rocky Mountain High Intensity Drug Trafficking Area, a network of law enforcement organizations in four western states that share information on drug-running patterns.

“If you look at those trends, you have to say there’s something going on here,” said Tom Gorman, the director of the group, which goes by the acronym RMHIDTA. “And it certainly appears that Colorado has become a source state for destinations east of here.”

The report is a preliminary one, Gorman said, and its figures are not comprehensive. The numbers come from a database kept by the Department of Justice’s El Paso Intelligence Center, to which law enforcement agencies voluntarily report seizure information.

Gorman said his organization did not try to compare Colorado’s marijuana exports to those from other states, most notably California. A RMHIDTA survey last year noted that Drug Enforcement Administration officials in St. Louis reported seizing for the first time more marijuana grown in Colorado than in California in 2011.

The report released Thursday is titled, “The Legalization of Marijuana in Colorado: The Impact.” In addition to the pot seizure statistics, it reports that adult and teen use of marijuana, emergency room visits related to marijuana and fatal crashes involving drivers testing positive for marijuana have all increased.

However, Mason Tvert, one of the proponents of the marijuana-legalization measure that voters passed in November, said the report’s title is misleading because almost all of the figures are pre-legalization. Tvert contended that the system of regulated pot shops the legalization measure will create will limit black-market diversion. Gorman, Tvert noted, campaigned against legalization.

“There is a reason we don’t see people brewing beer and distilling spirits in their basements and bootlegging it around the country,” Tvert said. “It’s because we abandoned alcohol prohibition and decided it made more sense to regulate it. We can do the same thing with marijuana in Colorado.”

John Ingold: 303-954-1068, jingold@denverpost.com or twitter.com/john_ingold