Attorney General Jeff Sessions is reinforcing threats to go after sanctuary cities, warning the administration may withhold federal crime-fighting funds for four places struggling with gun violence.

It is the latest threat made by Sessions in his public campaign to force cooperation between local authorities and federal immigration officials.

The Justice Department sent letters to police departments in Baltimore, Md., Albuquerque, N.M., Stockton and San Bernardino, Calif. telling them if they wanted federal help to root out drug trafficking and gang crime, they’d have to work with federal immigration authorities.

Among other things, the Justice Department said they must give the authorities access to jails and provide advance notice before releasing someone in custody who is wanted on immigration violations.