CHICAGO (AP) — Chicago police have started checking the identities of people attempting to enter four blocks on the city’s West Side to discourage gatherings of gang members.

Under the order, only people who live on the blocks being targeted would be able to enter them. Police spokesman Anthony Guglielmi said Tuesday the order was issued to disrupt the open-air drug markets and the violence on the blocks.

The American Civil Liberties Union says the move may be unconstitutional and police can use other means to stymie criminality.