The investigation had now been widened to include "everyone who sat on the supervisory board of Porsche's holding company between March and October 2008, as well as a former employee," a spokesman for prosecutors said.
That means that Ferdinand Piech, supervisory board chief of Europe's biggest carmaker Volkswagen, Porsche supervisory board boss Wolfgang Porsche and works council head Uwe Hück are now implicated in the probe.
The accused are being investigated on suspicion of "complicity to manipulate the market," the spokesman added.
In December, prosecutors charged Porsche's former chief executive Wendelin Wiedeking and former finance chief Holger Härter with manipulating Porsche's share price in connection with the – ultimately unsuccessful – attempt to take over Volkswagen.
Wiedeking and Härter allegedly made false statements in public with regard to Porsche's intentions in 2008. Both Wiedeking and Härter deny the charges.
The prosecutors argue that Porsche issued at least five public statements between March and October 2008 publicly denying the plans, even though preparations for the takeover were already underway.
It was not until several months later that Porsche revealed in a surprise statement its takeover plans, sending the share price rocketing.
Porsche's bid to acquire the much-bigger VW eventually unravelled, leaving the luxury sports carmaker with more than €10 billion in debt.
The two companies then made a fresh attempt to tie the knot in 2009, with VW initially acquiring 49.9 percent in Porsche in the first stage of the complex takeover agreement, the completion of which ran into a number of legal and tax hurdles.
Last year, a German court rejected two compensation claims brought against Porsche by investors who felt duped by the company over the tie-up.