US Supreme Court Justice Samuel Alito has indefinitely extended a stay blocking the enforcement of a Texas law that criminalizes illegal entry into the state from other countries. Monday’s extension came just as a previous extension issued last week was set to expire.

In late February, a federal judge issued a preliminary injunction preventing the law from being enforced, noting that the Supremacy Clause of the US Constitution and US Supreme Court precedent place immigration matters under federal jurisdiction. The US Court of Appeals for the Fifth Circuit later stayed that injunction, allowing the law to go into effect. Justice Alito stayed the Fifth Circuit’s order on March 4 without explaining his reasoning.

This latest extension does not have an expiration date. Only Alito or the whole court can vacate the extension.

Texas’s law, SB 4, makes it a misdemeanor offense for foreign nationals to enter the state from abroad unless arriving at a “lawful port of entry,” upgrading the offense to a felony for subsequent convictions. It also gives magistrates the power to order persons found violating the law to return abroad in lieu of prosecution.

The law came under quick scrutiny amid a battle between the state and the federal government concerning irregular migration on the US’s southern border. Opponents, including the US Department of Justice, argue that the state overstepped in passing the law as immigration is normally a matter of federal law. However, Texas contends that the law is necessary for the state’s “right to defend itself,” accusing the Biden administration of failing to address an “invasion” at the southern border. The law is part of a Texas initiative to curb irregular migration called “Operation Lone Star.”

In January, the Supreme Court voted 5-4 to allow federal border patrol agents to cut Texas-installed razor wire fencing along the US-Mexico border after the Biden administration argued that the fencing prevented agents from reaching migrants who had crossed into the US.

Previously, in December, a three-judge Fifth Circuit panel ordered Texas to remove its floating barriers in the Rio Grande, which forms part of the US-Mexico border. The whole Fifth Circuit is now reconsidering that decision.

The US Supreme Court struck down most portions of an Arizona immigration law that criminalized undocumented immigrants’ presence in the state in 2012.