Troubled blood-testing startup Theranos says it will continue serving customers through independent retail locations a day after the nation's largest drugstore chain severed ties with the company.

Walgreens said Sunday that it will immediately close all 40 Theranos Wellness Centers at the drugstore chain's Arizona locations. In January, Walgreens Boots Alliance Inc. told Theranos to stop sending samples collected at its stores to a testing facility that drew regulatory scrutiny over possible patient risks.

Palo Alto, California-based Theranos has raised millions of dollars by pitching its technology as a cheaper, faster way to run blood tests.

The privately held company said in April that it was under investigation from several regulators and agencies. That followed a series of Wall Street Journal reports that raised questions about the company's tests.