Russia has a long history of spying on the U.S. from the relative safety of Mexico City, dating back to the 1980s. Decades before that, Stalin had his chief rival assassinated there.

Russian intelligence services are building up their presence in Mexico for spy operations targeting the United States, a return to Cold War tactics by an increasingly aggressive regime, according to U.S. officials and former intelligence officers.

Russia has added dozens of personnel to its embassy staff in Mexico City in the past few years, even though Moscow has only limited trade ties with the country. U.S. officials say the trend is concerning and believe the extensive buildup is aimed at bolstering the Kremlin’s intelligence operations targeting the U.S., as well as its propaganda efforts aimed at undermining Washington and Ukraine.

The Biden administration has raised the issue with the Mexican government, a U.S. official told NBC News. “Russia has really invested in Mexico in terms of seeking to extend their presence,” the official said.