U.S. efforts to battle powerful drug cartels from inside Mexico have ground to a halt since January as strained relations between the two countries have frozen attempts to corral drug kingpins, according to current and former senior officials in both nations.

Until recently, U.S. and Mexican authorities routinely, if cautiously, shared intelligence on major cases. But in December, Mexico enacted a law requiring U.S. authorities to report their law-enforcement contacts in the country to the Mexican government, which American investigators widely view as corrupt. The new policy has led investigators on both sides of the border to pause their cooperation, fearful that the new disclosure rules could compromise cases - or worse, get informants or Mexican officials helping the Americans killed.

On-the-ground operations, including raids on Mexican drug labs, have largely ceased and U.S. authorities are now struggling to track movements of U.S.-bound cocaine from Venezuela and Colombia through Central America and into Mexico, according to two sources familiar with the matter. Some U.S. drug agents working in Mexico reported that they had been tailed by local police, raising alarms about their safety. And dozens of U.S. law enforcement agents can’t get visas to work in Mexico.

“Most of our most important cases are at a standstill,” a senior U.S. law enforcement official told Reuters. “If we have to report our sources to their foreign ministry, it jeopardizes our sources and methods. The system is set up intentionally now so that Mexican law enforcement can’t help us.”