MACUNGIE, Pa. — Thousands of Mack Truck workers went on strike Sunday at plants in three states, the first such walkout in decades.

United Automobile Workers union members began picketing at the Mack Truck cab and vehicle assembly plant in Macungie, Pa., where the company says “every Mack truck built for the North American market gets its start.”

Workers also walked out of plants in Middletown, Pa.; Baltimore; Hagerstown, Md.; and Jacksonville, Fla., the union said. In all, the company said the strike involves about 3,500 employees.

“One of the biggest issues for us is the job security,” said Walter Smith, president of the local representing Pennsylvania workers. The union said many issues are unresolved, including wages, job security and pension and health benefits.

Union officials say they will be available to reconvene negotiations Oct. 21, eight days after the beginning of picketing.

Mack Trucks President Martin Weissburg said in a statement that the two sides had been making progress and have a “positive working relationship.”

Officials remain confident they can reach an agreement that “provides a competitive wage and benefit package for our employees and families, and helps to ensure the company's competitiveness,” he said.