Ruh-Roh. Carl Icahn Has a Message for Apple’s Tim Cook.

Activist investor Carl Icahn plans to send an open letter to Apple Chief Executive Tim Cook on Thursday that he promises “will be interesting.”

Not much more is known beyond the message/threat.

https://twitter.com/Carl_C_Icahn/status/519918577718423552

As recently as August, Icahn boosted his stake in the company to 45 million shares, according to a filing with the U.S. Securities and Exchange Commission.

Icahn’s intentions are not immediately clear. He could not be reached for comment.

And in case anyone had forgotten, Icahn reminded his Twitter followers that since he announced his large position in Apple and demanded more stock buybacks, shares of the iPhone maker have risen over 50 percent.

https://twitter.com/Carl_C_Icahn/status/519918488040005632

A year ago, Icahn began pushing Apple to take on debt to execute a $150 billion stock-buyback program. The billionaire investor even met with Cook over dinner in New York to press his case.

Icahn went so far as to file a shareholder proposal in November 2013, saying Apple should amp up its buyback program. He ultimately withdrew it after Apple increased its stock dividend.

No one knows what, precisely, Icahn is after this time around, but whatever it is, expect a very public explanation, both in the open letter and probably on television as well. Icahn needs the bully pulpit to lay out his case, because Apple, with its $600 billion market cap, is simply too large for him to acquire the kind of sizable stake he’d need to force the issue.

This article originally appeared on Recode.net.