Cerberus switched to a 13D filing from a 13G late on Monday, signaling plans to seek change or influence control at Frontier, including through possible transactions.

Activist investor Jana Partners said on Oct. 17 it had built a position in Frontier and urged the third-largest U.S. fiber broadband provider to sell itself.

Another investor, a large unidentified communications company, has partnered with Jana in investing in Frontier, Jana said.

Cerberus did not immediately respond to a request for comment.

Ares Management, Frontier's biggest investor with a 16% stake, also updated its regulatory filing earlier in the year, saying it plans to seek change or influence control at Frontier.

Frontier's stock price has climbed 25% in the last month as speculation about the company's future increased. Since January, the share price has dropped 21% on concerns about its debt-funded fiber build-out strategy.

Fiber providers have been popular targets of private equity firms and infrastructure investors, including aggressive buyers like KKR, Apollo Global, Searchlight Capital Partners, Macquarie Infrastructure Partners, EQT, Oak Hill and Ares Capital.

(Reporting by Svea Herbst-Bayliss; Editing by Richard Chang)

By Svea Herbst-Bayliss