Mt Gox's heyday coincided with Silk Road, the "dark web" marketplace for drugs, weapons, and more that used bitcoin as its main currency.
Silk Road was shut down by the FBI in October 2013 when its founder Ross William Ulbricht was arrested.
Karpeles was asked how big Silk Road's influence over Mt Gox's exchange was, as at the time it was the main exchange to buy and sell bitcoin on.
He wrote: "We didn't have an exact estimate, but following Ulbricht's arrest and shut down of silk road we noticed a drop in activity which allowed put a figure on that at around 20%~30%."
Ulbricht, who was 29 when he was arrested, was sentenced to life in jail for drug trafficking. Asked about the punishment, Karpeles wrote: "Evidence seems clear enough it was him, and there is, of course, a need to make an example of him as first ever such case, but it's sad for someone that young to have to spend life behind bars."
Karpeles was also asked about what Mt Gox did to stop child traffickers who accepted bitcoin through sites such as Backpage.com.
"MtGox cooperated as much as possible with law enforcement when presented with valid requests that did not infringe privacy of the MtGox userbase in general," he wrote.