A Mexican company’s network tests disrupted emergency communications in El Paso, Texas, shortly after this month’s mass shooting, according to U.S. Senator Ted Cruz, who spotlighted a long-simmering cross-border dispute over wireless interference.

Altan Redes, the wireless operator, performed tests in Ciudad Juarez that interfered with communications in El Paso two days after the shooting, Cruz said in a letter to U.S. Federal Communications Chairman Ajit Pai and Secretary of State Mike Pompeo. The tests affected fire and rescue services, including defibrillators and other ambulance equipment that communicates with hospitals, the Texas Republican said.

Ted Cruz
Photographer: Andrew Harrer/Bloomberg

In a response to Cruz,...