"These negotiations have taken a turn for the worse", UAW's letter, signed by Vice President Terry Dittes, said.
But the talks quickly bogged down the next day as two sides clashed and GM's negotiators rejected a series of union proposals, leaving the talks deadlocked and prompting an icy response from Dittes, who accused the company of a lack of professionalism.
"We, in this union, could not be more disappointed with General Motors", Dittes said.
GM leads all companies in automobiles produced in Mexico at just over 833,000 a year ago, according to LMC and the Center for Automotive Research, a think-tank based in Ann Arbor, Michigan. GM responded on Sunday with a counteroffer that UAW that was reportedly nearly identical to a previously-rejected offer.
The update follows news that both sides began to make good progress in hashing out agreements on a number of issues in a big step toward a tentative agreement.
"We continue to negotiate in good faith with very good proposals that benefit employees today and builds a stronger future for all of us. We are committed to continuing discussions around the clock to reach a resolution", GM said in a statement.
"You didn't even have a professional courtesy to explain why you could not accept or why you rejected our package proposal for each item addressed", Dittes wrote in his letter. GM has already halted work at an assembly line in Lordstown, Ohio, as well as transmission plants in Warren, Michigan, and Baltimore.
The UAW sent out an urgent letter to leaders Sunday morning that outlined additional trouble in ongoing contract talks. Of GM vehicles sold in the USA, 22% are produced in Mexico.
The strike immediately halted production at GM's USA factories. Parts shortages also have forced the company to close plants in Mexico and Canada.
Wages and pensions are among the issues that remained unsettled. However, with the strike moving into its fourth week, those losses could end up spiraling to $90 million a day, Anderson said. "They brought down some pizzas, some chicken wings and they spent some time out on the picket line with us", said Lynn Nelson, Local 2164 Vice President.