WASHINGTON (CNN) -- The Justice Department said it won't try to block state laws that legalize marijuana and issued loosened enforcement guidelines for federal prosecutors intended to focus on serious trafficking cases.

Attorney General Eric Holder, in a conference call Thursday morning, notified the governors of Colorado and Washington that the department, for now, will not seek to pre-empt those states' laws, which followed voters' approval of ballot measures that legalized recreational marijuana use.

Marijuana will remain illegal under the federal Controlled Substances Act. But the attorney general issued a memo to federal prosecutors in all states tightening federal marijuana prosecution standards.

Under the guidelines, federal prosecutors are required to focus on eight enforcement priorities, including preventing marijuana distribution to minors, preventing drugged driving, stopping drug trafficking by gangs and cartels, and forbidding the cultivation of marijuana on public lands.

The new guidelines have been months in the making and took on some urgency after citizens in Colorado and Washington approved the ballot measures last fall. Nineteen states and the District of Columbia allow some legal use of marijuana, primarily for medicinal purposes.

The attorney general told the Washington and Colorado governors that the Justice Department will work with the states to craft regulations that fall in line with the federal priorities, and reserves the right to try to block the laws if federal authorities find repeated violations.

The memo to prosecutors also seeks to address one common complaint from medicinal marijuana dispensaries in some states, which have been subject to raids by federal agents because they were deemed too big or profitable.

The size and profitability of marijuana businesses will still be a factor prosecutors can consider, but there also must be additional illegal activities for prosecutors to take action.

The new guidelines don't change federal money laundering rules, meaning that some large banks may still be leery of doing business with marijuana producers and sellers. However, Justice Department officials said there is some leeway for banks to provide services to such businesses, so long as they don't violate the eight priorities being assigned to federal prosecutors.