Homeland Security cannot promise that illegal immigrants will not be arrested if they come forward to report they have been a victim of a crime or a witness to one, a spokesman said Tuesday.

Some victims and witnesses themselves are potentially criminal immigrants who may pose a threat to the country, David Lapan, a spokesman from DHS said at a news briefing.

Lapan added that immigration arrest in courthouses are necessary because some jurisdictions will not cooperate with requests to alert federal agencies.

Los Angeles officials, for example, are already attributing a drop in reported crimes to President Trump's illegal immigration crackdown. These officials fear the threat of arrests can deter victims from reporting crimes or witnesses from cooperating in investigations.

Los Angeles Police Chief Charlie Beck said last month that his city has seen a 25 percent decrease in the number of sexual assaults reported by Latinos living in the city and a drop of about 10 percent in the number of reported domestic violence cases since Trump took office.

Homeland Security Secretary John Kelly and Attorney General Jeff Sessions also defended the practice of arresting these immigrants at courthouses in a letter last month to the chief justice of the California Supreme Court.

"Because courthouse visitors are typically screened upon entry to search for weapons and other contraband, the safety risks for arresting officers and persons being arrested are substantially decreased," Kelly and Sessions wrote.

The Associated Press contributed to this report.