Eleven senators have sent a letter to the head of the Department of Health and Human Services, demanding answers to a series of questions about the Centers for Disease Control and Prevention’s nonfatal firearm injury estimates. The inquiry relies on the findings of an investigation by The Trace, a nonprofit news organization covering gun violence in America, and FiveThirtyEight, and is led by Democratic Sen. Bob Menendez of New Jersey.

The Trace and FiveThirtyEight first reported last year that the CDC’s 2016 gun injury estimate was so uncertain the agency classified it as “unstable and potentially unreliable.” Since then, the agency’s data, which in 2017 was derived from a sample of just 60 hospitals, has become even more unreliable. The CDC’s gun violence estimates are widely cited in academic articles.

“Given that the CDC is not currently conducting gun violence research,” the letter reads, “the very least the agency can do is to ensure that its gun injury numbers are accurate.”

The letter asks HHS Secretary Alex Azar to explain the CDC’s methods for tracking nonfatal firearm injuries, the cause of its increasingly unreliable estimates, and whether the agency has undertaken any actions to improve the quality of its data. The senators also ask whether the Dickey Amendment — a piece of 1996 legislation that bars the CDC from using its funding to “advocate or promote gun control” — has played any role in the agency’s continued reliance on a data source that’s ill-suited for producing firearm injury estimates.

“We as lawmakers, as every American citizen, should be able to follow and understand the latest trends on firearms injuries without the concern of coming across ‘unstable and potentially unreliable’ data,” Menendez told The Trace in an email.

The CDC has previously acknowledged its estimates have a high degree of uncertainty. “CDC continues to look into various ways to strengthen the estimates for nonfatal firearm injuries,” said spokesperson Courtney Lenard in an email.

Researchers interviewed by The Trace and FiveThirtyEight believe the CDC’s estimates are too flawed to use. Guohua Li, editor-in-chief of the medical journal Injury Epidemiology and director of Columbia University’s Center for Injury Epidemiology and Prevention, said the estimates could be improved by drawing upon a larger and more reliable source of data, such as another database administered by HHS.

Menendez makes reference to this proposed solution in the letter, writing, “There appears to be no rational reason that the CDC and HHS use different databases.”

The letter was also signed by Democratic Sens. Cory Booker, Kirsten Gillibrand, Mazie Hirono, Richard Blumenthal, Amy Klobuchar, Kamala Harris, Tina Smith, Chris Murphy and Chris Van Hollen and independent Angus King. The letter asks Azar to respond by April 20.