Any city or town that violates that federal statute could lose some Justice grant funding this year, Sessions said, as long as Congress had already spelled out those conditions before the government awarded the grants. Sessions has taken steps to revoke federal funding from nine jurisdictions, each of which insists that it has complied with the law.
Sessions's memo came nearly a month after a federal judge in California halted Trump's initial attempt to strip funding from sanctuary cities, saying that only Congress, not the president, could impose new conditions on federal grants, among other concerns.
Later Monday, federal lawyers in that case filed a motion asking the judge to reconsider his ruling because they said Sessions's memo had addressed these and other issues. Sessions's memo does not require cities and towns to detain immigrants, they said, and will only apply to Department of Justice and Homeland Security grants. For now, the attorney general will only seek to withhold funds from three Justice department grant programs.
However, in his memo, Sessions warned that the administration and Congress in the future may seek to "tailor" grants, or impose additional conditions related to immigration on the jurisdictions that seek funding.
And under the language in the memo, the Justice Department could try to link the awarding of future grants to other factors, including whether local law enforcement officials comply with federal immigration efforts or refuse to detain immigrants for deportation
Sessions also made clear that the Trump administration would continue to publicly criticize noncooperative cities, states and towns, which he says threaten public safety by refusing to work closely with federal immigration officials.
"While the Executive Order's definition of 'sanctuary jurisdiction' is narrow, nothing in the Executive Order limits the Department's ability to point out ways that state and local jurisdictions are undermining our lawful system of immigration or to take enforcement action where state or local practices violate federal laws, regulations, or grant conditions," Sessions wrote.
The memo marks the first official guidance to federal grant issuers since the president's executive order in January threatened to strip federal funding from cities and towns that refuse to cooperate with federal immigration officials.
Hundreds of U.S. cities panicked when the order was signed, afraid that they would lose millions of dollars for education, health care and other programs if they refused to detain immigrants for deportation officials.
At least one local government, Miami-Dade County, reversed its policy as a result of the threat.
But others filed lawsuits and cheered last month when a federal judge in California temporarily halted that part of Trump's executive order, saying it was too broad.
The memo from Sessions will probably stir some relief among a broad swath of U.S. cities, including the nine that the Trump administration has labeled sanctuary jurisdictions so far. Those cities insist that they comply with USC 1373, a federal law that focuses on communication between federal and local officials and which does not mention immigration detainers.
Until now, no official definition of a sanctuary jurisdiction has existed. Estimates of the number of sanctuary cities has varied from more than 100 to about 600.
Trump and others say sanctuary cities pose a threat to public safety by refusing to hold immigrants in jail after they have been cleared for release from criminal custody, so that federal immigration officials can detain them for deportation proceedings.
Local officials counter that they do not have the legal authority to hold a person for a civil immigration proceeding after a judge in a criminal case has released them on bail. And they argue that Trump's policy is making cities less safe because immigrants are increasingly afraid to report crimes or serve as witnesses, for fear of deportation.
Matt Zapotosky contributed to this report.