States across the U.S. have added nearly three million records of people of dangerously mentally ill people to a national background check system since the 2012 Sandy Hook shooting, according to an analysis released Friday by Everytown for Gun Safety, a group that bills itself as the largest gun violence prevention organization in the country.

Between the December 2012 Sandy Hook shooting and December 2015, states submitted more than 2.3 million records to the National Instant Criminal Background Check System (NICS), the group found, based on data obtained through a Freedom of Information Act request. The group said that 400,000 more records have been added since that request.

Before the Sandy Hook shooting, the group said that 19 states had fewer than 100 records in the system and now only four states submitted fewer than 100 records: Alaska, Montana, New Hampshire and Wyoming. Everytown Research Director Ted Alcorn criticized these states, saying in a press release that "four states continue to lag considerably behind, leaving fatal gaps in the background check system that put all Americans at risk."

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Here's the breakdown for those states, according to Everytown:

Alaska (93 records submitted)

Montana (3 records submitted)

New Hampshire (2 records submitted)

Wyoming (4 records submitted)

Everytown said that Iowa, West Virginia and New York added the largest number of records since 2012.

After mass shootings over the last several years, many lawmakers, Republicans in particular, have pointed to mental illness as an issue to tackle to prevent future shootings. In response, Democrats have argued that it's not enough and have pushed for more gun reforms that focus especially on background checks.