Liberty has a 46.2% equity interest in Sirius, most of which was acquired when a $530 million loan from Liberty saved the company from bankruptcy in 2009. In a regulatory filing, Liberty said its planned conversion of almost half of its preferred stock in Sirius would boost Liberty's total to more than 32% of Sirius' outstanding common shares.

"As soon as practicable," the filing continued, Liberty would use this leverage to place a majority of directors on Sirius' board. The media investment firm headed by John Malone already controls five of the board's 13 seats.

Liberty's renewed commitment to control Sirius extends a drama that began on March 6 with the expiration of a standstill agreement related to the life-saving loan issued to Sirius three years ago. On March 20, Liberty applied to the Federal Communications Commission for de facto control of Sirius on grounds that the preferred stock obtained in exchange for the loan already represented a controlling interest of 40%.

The FCC dismissed Liberty's application on May 4, calling it "defective with respect to 'execution' and 'other matters of a formal character.' " The dismissal started the clock on a 30-day deadline for Liberty to file the amended application it announced on Thursday.

In addition to the preferred stock, Liberty obtained Sirius equity by entering into a forward purchase contract for additional shares, as disclosed on May 8, and through open-market purchases on May 8 and 9. In a Form 4 filed May 9, Liberty put the price of the forward contract at $2.15 per share and the open-market purchases between $2.126 and $2.137 per share.

Sirius responded with a filing of its own Thursday in which it acknowledged being "engaged in discussions with Liberty" over ownership. But it said that no agreement had been reached and that there was "no assurance" one would be.

Liberty's efforts to control Sirius had been resisted by CEO Mel Karmazin, who not only wants to run the operation unfettered by a majority shareholder but wants a control premium in any deal that transfers power. However, given that Sirius common closed Wednesday at $1.89 per share, Liberty could effectively argue that it has already paid a premium.