Prosecutors at a federal labor agency have determined that Amazon is a joint employer of subcontracted drivers who delivered packages for the company in California, pushing back on claims from the online retailer that they are not its employees.

The decision, made by a regional director for the National Labor Relations Board (NLRB) in Los Angeles, came after the agency investigated unfair labor-practice charges filed against the company by the Teamsters union.

The prominent labor group represents UPS drivers and has been seeking to unionize Amazon drivers. However, it has encountered challenges, most notably because the company doesn’t directly employ drivers but relies on thousands of third-party businesses that deliver millions of customer packages every day.

Currently, over 275,000 drivers are employed with these businesses, which are called Delivery Service Partners, or DSPs.

The Teamsters and other labor advocates have long said Amazon exercises great control over the drivers—including by determining their routes, setting delivery targets, and monitoring their performances—and should be classified as a joint employer.

Last year, the Teamsters said they unionized dozens of drivers who work for one DSP in Palmdale, California, called Battle Tested Strategies. The labor group filed several unfair labor-practice charges against Amazon after the company refused to negotiate a union contract with them.

On Thursday, NLRB spokesperson Kayla Blado said agency prosecutors made “merit determinations” on three of those charges, one being that Amazon and Battle Tested Strategies were joint employers of the drivers who work for the firm.