What tools do you use to test accessibility in CI?

Our company is working on a product for customer assistance that will get used across the web. We were a bit naive about accessibility at first, but we're now making a concerted effort to make sure our product is usable for everyone, and we know accessibility affordances help everyone, not just people with disabilities. It's a React app, so testing interactively is really important.

We're already using BrowserStack/Selenium to do some end-to-end testing and we'd love to augment that. I'm pretty sure asserting on `aria-label` isn't enough. What automated tools do you use to exercise ensure your apps remain accessible?