The Biden administration on Tuesday alerted Texas that it was opening a civil rights investigation into Gov. Greg Abbott’s (R) ban on school mask mandates, pressing its fight with Republican governors over their pandemic policies.

The U.S. Education Department had earlier announced that its Office of Civil Rights was investigating Iowa, Florida, Oklahoma, South Carolina, Tennessee and Utah — all Republican-led states where officials have barred school districts for requiring that staff and students wear masks despite high rates of hospitalization of children” with covid-19.

The letter sent to Texas, like the ones to the other states, said that the bans on mask mandates “may be preventing schools in Texas from meeting their legal obligations not to discriminate based on disability and from providing an equal educational opportunity to students with disabilities who are at heightened risk of severe illness from covid-19.”

The investigation was opened following the release of a Sept. 17 memo from the Texas Education Agency that says “school systems cannot require students or staff to wear a mask.”

Biden had said last month that the Education Department would take steps to prevent governors and state legislatures from banning school district mask mandates.

Those opposed to the mandates say wearing a mask is a matter of personal choice. But the Centers for Disease Control and Prevention and other public health officials say that wearing masks is a critical tool in the effort to spread the highly contagious delta variant of the coronavirus.

The letter to Texas, like the earlier ones sent to other states now being investigated, says that the nation has experienced “significant increases in the number of new covid-19 cases in the general population and specifically among school-age children.”

“Reports show distressingly high rates of hospitalization of children with covid-19,” it says. “National data also show that children with some underlying medical conditions, including those with certain disabilities, are at higher risk than other children for experiencing severe illness from covid-19.”