...Prieto, who last year helped organize unprecedented strikes at several dozen manufacturing plants in the northern border city of Matamoros, was charged with threats, inciting a riot, coercion and crimes against public servants. She has denied the charges.

With the United States-Mexico-Canada Agreement (USMCA) set to go into effect on July 1, U.S. unions are concerned the detention undermines Mexico’s pledge to protect workers’ rights, a central aim of the trade pact.

Prieto could remain in jail until late July as investigators probe the case, the Tamaulipas state attorney general’s office said following her June 8 arrest in Matamoros. A state judge rejected Prieto’s requests to be released while the case is pending.

“This clearly is intended to and will undoubtedly have a chilling effect on workers who are forced to make a life and death decision every day before they go to work under dangerous working conditions,” United Auto Workers union spokesman Brian Rothenberg said in a statement on Tuesday.

“It signals a very real challenge for the Mexican government to implement the reforms agreed upon by the U.S. and Mexico under USMCA.”...