NEW YORK (AP) — The mayors of the nation’s three largest cities are calling for Attorney General Loretta Lynch to crack down on some so-called “bad apple” gun dealers.

Bill de Blasio of New York, Eric Garcetti of Los Angeles and Rahm Emanuel of Chicago signed the letter to Lynch on Monday.

Their efforts were coordinated by the Brady Campaign and Center to Prevent Gun Violence.

The Brady Campaign says five percent of dealers supply the guns used in nearly 90 percent of crimes across the nation.

The mayors want Lynch to establish stricter guidelines for the dealers and step up investigations into their practices.

De Blasio said it was “a national problem that hit close to home” and noted that four New York police officers were slain this year by out-of-state guns.