Investigative Committee spokesman Vladimir Markin said Serdyukov's decision not to answer the panel's questions could mean his status as a witness might change because it could be considered intent to obstruct the inquiry, RIA Novosti reported.
"It is quite possible that Serdyukov's status in the case will change," Markin said.
Serdyukov's written statement was on the sale of the Defense Ministry's assets.
Serdyukov was fired Nov. 6 following a scandal over alleged real estate fraud involving about $130 million in state assets at Oboronservis, a Defense Ministry-controlled firm. Three people have been charged so far.
Markin said investigators will carry on the inquiry and plan to question more witnesses.
Serdyukov, ousted by President Vladimir Putin in November, appeared before investigators in December and declined to answer questions on the fraud case because his lawyer wasn't present, RIA Novosti said. His lawyer was reportedly ill.