Federal agents around the country were told in recent days that the F.B.I. would no longer investigate confrontations with immigration agents " cases that sometimes yield evidence that could be used to prosecute Department of Homeland Security agents implicated in violent encounters, according to people briefed on the decision. If enacted, such a change would sharply limit law enforcement scrutiny of immigration agents, as the Trump administration draws mounting criticism over killings at the hands of federal officers. In the last two weeks, two fatal shootings by Immigration and Customs Enforcement agents in Texas and Maine have thrust the government's tactics back into the spotlight.