Colorado law enforcement groups are raising concerns over ads for a marijuana convention that are on RTD buses across the city.

The Colorado Drug Investigators Association wrote in a letter last week to the Regional Transportation District board of directors that it worries that the ads — which promote the KushCon cannabis convention — send the wrong message.

“Advertising a marijuana conference, on the sides of Colorado’s main source of public transportation, will do anything but prevent further drug abuse,” Jerry Peters, the association’s vice president and an investigator with the North Metro Drug Task Force, wrote in the letter.

Peters asked that RTD remove the ads.

On Friday, Daniel Brennan, president of the Colorado Association of Chiefs of Police, said his organization was drafting a letter to send to RTD over the ads.

“We’re sending mixed messages, I think, to the public and to the youth on this,” said Brennan, who is also the Wheat Ridge police chief.

RTD spokesman Scott Reed said Friday the ads would stay up — although the transportation district told KushCon to modify them slightly to better reflect that they are paid advertisements.

“It’s an ad for an event that is being legally held at the Colorado Convention Center,” Reed said. “There should be no implication of support or endorsement for that event.”

RTD policy prohibits ads that tout illegal products or services. Because marijuana distribution is illegal federally, Reed said, RTD does not allow ads for medical-marijuana dispensaries even though medical marijuana is legal in Colorado.

But KushCon — despite billing itself as a “cannabis lifestyle” convention with appeal to marijuana enthusiasts — is a legal event that will not include marijuana on site. Bob Selan, chief executive of Dbdotcom, which publishes the marijuana-centric Kush Magazine and is sponsoring the convention, said the event’s main purpose is to provide information to medical-marijuana patients and other curious people.

“I think the signs on the buses are pretty harmless,” Selan said. “They’re saying, ‘Have a Kush Day!’ They’re not saying, ‘Go smoke marijuana.’ “

The convention runs Friday through Dec. 19. It will feature marijuana-growing and cooking classes, speeches, cannabis business booths, a skateboarding demonstration and concerts.

Law enforcement officials worry that what they see as promotion of marijuana will lead to negative social consequences, such as an increase in drug-impaired driving or teen drug use.

The officials say the ads are evidence that groups concerned about the increased visibility of marijuana in Colorado are losing the message battle.

“From a prevention and education perspective,” Brennan said, “we have a lot of work to do.”

John Ingold: 303-954-1068 or jingold@denverpost.com