MOSCOW — Former Russian Defense Minister Anatoly Serdyukov refused to answer questions from investigators on Friday in a $100-million fraud case that has snowballed into the biggest corruption scandal since President Vladimir Putin's return to the Kremlin.

Serdyukov, who was fired by Putin on Nov. 6, remained silent because his lawyer was unable to attend due to illness, Vladimir Markin, a spokesman for the Investigative Committee, said in a statement.

He will be called in again for questioning on Jan. 11 and remains a witness in a case that is seeking to prosecute his former subordinate Yevgeniya Vasilyeva for fraud and embezzlement over several deals involving Defense Ministry property.

The case, one of several sleaze scandals to flare up since Putin's return to the Kremlin for a third term as president in May, has raised questions over how much Serdyukov knew about the suspected fraud.

Vasilyeva was charged on Nov. 23 over a string of cut-price deals in which prime real estate was allegedly sold off to insiders at a loss to the taxpayer of at least 3 billion rubles ($98 million).

Serdyukov's relationship with Vasilyeva, 33, has also been the subject of speculation after reports he opened the door to police in his bathrobe and slippers when they launched an early-morning raid in October on her Moscow apartment, which is in the same building as his own.

The  former minister is the son-in-law of former Prime Minister Viktor Zubkov, a long-time political ally of Putin. A source close to the government has said Serdyukov may have paid the political price for his reported behavior.

Serdyukov's lawyer, Genrikh Padva, told legal newswire Rapsi that he had fallen ill and was unable to attend the questioning at the Investigative Committee, Russia's equivalent to the FBI.

Padva is one of Russia's most renowned defense and human rights lawyers, and previously represented Mikhail Khodorkovsky, the oil tycoon jailed in 2005 for fraud. Khodorkovsky is expected to walk free in 2014 after the sentence on a second conviction was cut on appeal.

Khodorkovsky's oil company, Yukos, was bankrupted by a series of controversial back-tax claims pursued by Russia's Federal Tax Service, which was headed by Serdyukov from 2004 until 2007, when he was named defence minister.