U.S. Attorney John Walsh said Thursday evidence that medical marijuana is having a detrimental impact on Colorado kids spurred his decision to crack down on dispensaries near schools.

Walsh, in the strongest federal action against dispensaries in Colorado to date, sent letters last week to 23 dispensaries within 1,000 feet of schools. The letters ordered the dispensaries to close by Feb. 27 or face potential criminal prosecution or seizure of assets, even though those dispensaries are operating in compliance with state law.

On Thursday, Walsh said state law plays a part in deciding how to use his prosecutorial discretion with medical- marijuana businesses, which are strictly illegal under federal law. But Walsh said protection of important federal interests takes precedence.

“One of those interests, without question, is protecting drug-free zones around schools,” he said.

Walsh said his office is working to identify any other dispensaries near schools and said the crackdown may extend to other types of medical-marijuana businesses — such as cultivation operations — that are close to schools.

What prompted the decision to send the letters, Walsh said, was evidence that medical marijuana is being diverted to youths. For instance, a report published last year by Denver Health drug-treatment doctor Christian Thurstone found that, of 80 kids admitted for substance-abuse treatment, nearly half said they had received marijuana from someone with a medical-marijuana license.

Walsh also mentioned a rising number of drug-related school suspensions, expulsions and law enforcement referrals. In the 2008-09 school year, before Colorado’s boom in medical-marijuana dispensaries, 534 students were expelled for drug violations, according to state Education Department data. In the 2010-11 school year, that number was 767.

“The threat to kids . . . came up to the top of the list,” he said.

Medical-marijuana advocate Brian Vicente said Walsh’s actions are a “federal overreach” and said Walsh should leave the policing of medical- marijuana businesses up to the state. He said Walsh hasn’t presented evidence that dispensaries are providing marijuana to kids or responsible for the increase in expulsions.

“Colorado has done a remarkable job in regulating the sales of medical marijuana at both the state and local level,” Vicente said. “I think the federal government needs to respect that.”

Walsh said he has been measured in his approach to dispensaries but said he does not have a defined list of criteria dispensaries must follow to avoid federal prosecution. He said he considers such things as whether dispensaries are targeting kids, engaged in money laundering, connected to organized crime, involved in selling marijuana out of state or linked to firearms and violence when deciding whether to prosecute the businesses.

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