Prosecutors say that they found $173M worth of signed checks made out to family and friends; indicating that Madoff was attempting to keep remaining money from investors.
From the AP article:
In the filing, Assistant U.S. Attorney Marc Litt said Madoff cannot be
trusted because he had long engaged in a "scheme that required the
defendant to lie routinely to thousands of people and a scheme which
has caused extraordinary damage to individuals, families, and
institutions all over the world."
The full article can be read here.