BOSTON - A group of U.S. senators, including Elizabeth Warren and Ed Markey of Massachusetts, is still seeking answers on federal barriers to research on medical marijuana and hit U.S. agencies for a lack of coordination with states on the issue.

Warren, Markey and six other senators sent a letter in July asking for information on federal agencies' efforts on medical marijuana research. Medical marijuana is currently legal in 23 states, including Massachusetts and Washington, D.C.

In a joint response sent to the senators in October, the heads of the Department of Health and Human Services, the Office of National Drug Control Policy and the Drug Enforcement Administration noted that a key research barrier was lifted in June 23.

"This action may facilitate more research into the potential therapeutic benefits of marijuana and its components by removing overlapping requirements with [Food and Drug Administration's] review process for investigational new drugs," the officials wrote.

"Recognizing our departments have distinct, but interconnected roles in this space, the relevant Federal agencies are planning to convene an interagency meeting to review additional actions that may be taken to encourage marijuana research," they added.

The Drug Enforcement Agency, headed by Michael Botticelli, has 265 researchers registered to "conduct bona fide research with marijuana and marijuana extracts," their letter said.

But the eight senators - Warren, Markey, Barbara Mikulski of Maryland, Barbara Boxer of California, Jeff Merkley and Ron Wyden of Oregon, Kirsten Gillibrand of New York and Cory Booker of New Jersey - said they were disappointed with the response and asked for more information from the agencies by Jan. 31, 2016.

Federal state regulatory barriers to marijuana research persist, they wrote.

"This regulatory scheme which limits medical marijuana research is outdated and in desperate need of serious and immediate review," the senators' letter said. "These problems are exacerbated by a lack of coordination between the agencies and states."

New England Treatment Access has opened the first Western Massachusetts medical marijuana dispensary in Northampton. Here are some of the varieties currently available. (Don Treeger / The Republican)

The efforts to reduce the barriers are "time-sensitive" and warrant "immediate attention from federal agencies," because patients and families want safe therapies available as quickly as possible, the letter added.

The senators asked for more information on the supply of marijuana for research purposes, since the University of Mississippi has the only bulk-manufacturing permit granted by the DEA. They asked for a breakdown of all strains at the University of Mississippi.

The senators also asked how the federal agencies are planning to allow researchers to use the existing supply of marijuana in the states that have legalized marijuana for both medical and recreational use.

Massachusetts voters legalized medical marijuana in 2012. A group of activists is pushing to legalize marijuana in Massachusetts for recreational use through an initiative on track for the 2016 statewide ballot.

"We must ensure that our public policy regarding medical marijuana is guided by the best science available, and in the absence of that science we must ensure that we are taking swift and deliberate steps to facilitate the research that provides for it," the senators wrote in their letter.