House Oversight Committee chairman James Comer (R., Ky.) is launching an investigation into the sanctuary city policies of four heavily Democratic cities. Comer is writing letters Monday to the mayors of Boston,

House Oversight Chairman James Comer announced probes Monday into four sanctuary cities, calling on them to come before Congress and defend themselves.

Comer sent the requests to the mayors of Boston, Chicago, Denver and New York City, and invited the mayors to testify in front of the committee on Feb. 11.

Denver's mayor, police and chief prosecutor are standing firm against working with federal immigration authorities to fulfill President Trump's controversial deportation plan. Why it matters: The U.S.