LISLE, Ill. (AP) — Billionaire investor Carl Icahn recently boosted his stake in Navistar, taking advantage of the recent steep drop in the struggling heavy truck and engine maker's stock price, according to a regulatory filing.

Icahn now owns about 13.2 percent of the Lisle, Ill.-based company's common stock, up from his previous 11.9 percent stake, Navistar said Thursday in a filing with the Securities and Exchange Commission.

A day earlier, Navistar said in an SEC filing that the activist hedge fund MHR Fund Management LLC, the company's biggest shareholder, had increased its stake to almost 15 percent. MHR is led by Mark Rachesky, a former Icahn protege. Last year Rachesky thwarted Icahn's attempt to take over Lions Gate Entertainment Corp.

For its part, Navistar announced in late June that it adopted a stockholder rights plan designed to protect shareholders in the event of a hostile takeover attempt.

The moves aren't surprising given the dramatic drop in Navistar's stock over the past few days, which began after the company said Friday that it was in talks with federal regulators on a plan that will allow it to continue shipping trucks while it makes a transition to a new emission-reducing technology that will bring it into compliance with federal requirements.

Navistar didn't provide any financial details of the plan, except to say that it will require more product development, resulting in additional short-term costs. That spooked investors and Navistar's shares plunged, closing down 15 percent that day. The shares have continued to drop in the day since, losing a total of 20 percent of their value since the news first broke.

Navistar shares fell 10 cents to $22.90 in premarket trading Thursday.