An Arizona lab run by blood-testing firm Theranos Inc. put patients at risk and failed to quickly fix its deficiencies, the main U.S. lab regulator found, triggering a new round of sanctions last month against the company.

The Centers for Medicare and Medicaid Services imposed some of the harshest penalties in its arsenal on the Arizona lab. The agency revoked the lab’s U.S. testing license, barred it from billing Medicare and ordered it to alert customers of its problems, according to a Jan. 27 letter obtained by The Wall Street Journal in a public records request.