ST. LOUIS (AP) — A Black mechanic for the company that provides school bus services for the St. Louis school district said he found a noose at his workstation, leading at least 100 drivers to stop work in a show of support.

The work stoppage began Monday and continued Tuesday for St. Louis drivers employed by Missouri Central School Bus. Most after-school activities in St. Louis Public Schools were called off both days. And 56 bus routes were uncovered Tuesday morning, forcing parents to make other plans.

“The allegations that surfaced Friday from the Missouri Central bus depot are upsetting, and it is our hope that management at Missouri Central will get to the bottom of what is clearly unacceptable behavior,” a statement from St. Louis Public Schools said. It also urged the company and its drivers to find “common ground” to resolve the stoppage.

“The families of Saint Louis Public Schools should not be the ones left suffering in this situation,” the statement said.

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Mechanic Amin Mitchell said he found a noose last week at his workstation. Mitchell told the St. Louis Post-Dispatch that he believed the noose was meant to send a racist message to intimidate him after an argument with a manager over Mitchell’s concern that some bus brakes were inadequate.

Mitchell posted social media video of the noose, fashioned from a thin rope and lying on the floor in the area where he works.

“That’s a message that says, ‘If you don’t stop doing what you’re doing, something bad is going to happen right away,’” Mitchell told the newspaper. He didn’t immediately return messages from The Associated Press seeking comment.

Missouri Central said in a statement that it will hire an independent third party to investigate claims by Mitchell and others of racism.

“At Missouri Central, our policy is to provide and foster a work environment that is welcoming to all regardless of age, race, ethnicity and sexual orientation,” the statement said. “There is zero tolerance for any behavior that violates this policy.”

The state, city and county NAACP chapters called Tuesday for a federal or state investigation.

“The noose is a symbol of hate and sends a clear message of racial terror and the potential for violence,” Missouri NAACP President Nimrod Chapel Jr. said in a text message.

The drivers are members of Laborers’ International Union of North America. Because their contract does not permit strikes, drivers told the Post-Dispatch, they called in sick with “personal issues.”