The explosive growth of so-called "sanctuary cities," under fire Wednesday by President Trump, has reached over 300 and blocked federal officials from seizing, and likely deporting, more than 17,000 criminal illegals.

The Center for Immigration Studies, which has collected the names of every sanctuary, reports:

"Over the 19-month period from January 1, 2014, to September 30, 2015, more than 17,000 detainers were rejected by these jurisdictions. Of these, about 11,800 detainers, or 68 percent, were issued for individuals with a prior criminal history."

"Detainers" are requests from Immigration and Customs Enforcement to city and county law enforcement to hold a suspected illegal criminal for federal arrest. Cities around the country, from Washington, D.C., to San Francisco, refuse the detainers, claiming that it's unfair to remove illegal immigrants.

However, in some cases, those the cities and counties let go commit other horrible crimes including drunk driving deaths and murder.

Near the end of the Obama administration, and under pressure from House Republicans, sanctuary cities were warned that if they continued to block ICE, they would lose federal funding for jails and law enforcement. Several, however, ignored the threat.

On Wednesday, Trump is expected to put more pressure on the law-breaking jurisdictions.

CIS Policy Director Jessica Vaughan explained the issue in a recent post that accompanied her map of sanctuaries:

These cities, counties, and states have laws, ordinances, regulations, resolutions, policies, or other practices that obstruct immigration enforcement and shield criminals from ICE — either by refusing to or prohibiting agencies from complying with ICE detainers, imposing unreasonable conditions on detainer acceptance, denying ICE access to interview incarcerated aliens, or otherwise impeding communication or information exchanges between their personnel and federal immigration officers.

A detainer is the primary tool used by ICE to gain custody of criminal aliens for deportation. It is a notice to another law enforcement agency that ICE intends to assume custody of an alien and includes information on the alien's previous criminal history, immigration violations, and potential threat to public safety or security.

Paul Bedard, the Washington Examiner's "Washington Secrets" columnist, can be contacted at pbedard@washingtonexaminer.com