Five more corporate board members resigned after being flagged by the Department of Justice (DOJ) for potentially violating the antitrust provisions of the Clayton Act.

A total of 13 board members have now resigned, including those announced Thursday by the DOJ in a press release.

Companies have been on notice since an April 2022 speech by Assistant Attorney General Jonathan Kanter warning the DOJ intended to step up its enforcement of Section 8 of the Clayton Act, which prohibits directors and officers from sitting on boards of competing businesses.

“Enforcement of Section 8 will continue to be a focus for the division just as Congress intended,” said Kanter, who leads the DOJ’s Antitrust Division. “We will continue to enforce the antitrust laws when necessary to address illegal board interlocks.”

In one example included in the agency’s release, a director served simultaneously on the boards of three companies: Qualys, SumoLogic, and F5.

In another example, representatives of the investment firm Thoma Bravo sat on the boards of N-able, Dynatrace, and SolarWinds, the DOJ said.

Insurance companies Brookfield Asset Management and American Equity Investment Life Holding Company changed course when a Brookfield subsidiary had the contractual right to appoint a director to AEL’s board, but the DOJ raised concerns regarding a potential interlock.

In August, an Apollo Global Management-led investment group proposed acquiring all of Atlas Air Worldwide Holdings’ outstanding shares. At the time, two Apollo-affiliated individuals sat on the board of directors of Sun Country Airlines Holdings. Both Atlas Air and Sun Country provide crew, maintenance, and insurance for domestic air freight routes. The two Apollo-affiliated directors resigned from the Sun Country board after the DOJ raised red flags.