The Federal Trade Commission is likely to sue Amazon.com Inc. later this month, capping a four-year antitrust investigation into the company, people familiar with the matter said Tuesday.

The antitrust suit is expected to target the online retail giant’s popular marketplace, where third-party merchants, who now account for more than half of the company’s online sales, pay a commission on each sale, according to the people, who asked not to be named discussing the potential suit. Merchants have complained that Amazon unfairly ties access to its marketplace with its logistics service.

The long-expected complaint will be the fourth the agency has filed this year targeting Amazon, stepping up pressure by the Biden administration, which has focused on antitrust and competition as a keystone of its economic policy. FTC Chair Lina Khan, Biden’s pick to lead the agency, has long had the online commerce giant in her sights, writing a seminal paper as a law student about how to rethink the antitrust laws in connection with its online platform.

Top company executives met with the FTC’s three commissioners in mid-August to discuss the suit, though no settlement was discussed, according to the people.

In May, the agency sued the e-commerce giant in two separate cases for failing to delete data about children collected by its Alexa speakers and illegally spying on users of its Ring doorbells and cameras. Amazon said it disagreed with the FTC’s allegations, but agreed to pay $30.8 million to resolve the cases.

One month later, the FTC again sued Amazon in a consumer protection case, alleging the company duped consumers into signing up for Prime membership and deliberately made it hard to cancel. Amazon denies the allegations and that suit is ongoing.

The Wall Street Journal reported earlier that the new suit is expected this month.