Elon Musk has sent an additional letter of deal termination to Twitter after the world’s richest person subpoenaed a whistle-blower seeking documents on how the social media company measures spam accounts.

Musk walked away from a US$44-billion offer for Twitter in July, saying the company misled him and regulators about the true number of spam or bot accounts on the microblogging platform.

According to a court filing on Monday, Musk has sought information from whistle-blower and former head of Twitter’s security, Peiter Zatko, mostly about the way the microblogging site measures spam accounts.

A famed hacker known as “Mudge”, Zatko said in his complaint that became public last week that the company falsely claimed it had a solid security plan and prioritised user growth over reducing spam.

Following the disclosure, Musk’s legal team said allegations on certain facts, which were known to Twitter prior to 8 July but were not disclosed to them, have come to light that provide additional and distinct bases to end the deal.

Read: Twitter faces legal and political peril in whistle-blower case

The subpoena to Zatko comes in the run-up to a five-day trial in a US court set to begin on 17 October.

While Musk wants to terminate the deal, Twitter is asking the court to order him to buy it for the agreed $54.20/share.

Read: Twitter should have been a protocol, not a company: Dorsey

Meanwhile, an additional termination notice dated 29 August was delivered in case the 8 July notice was determined to be invalid for any reason, according to a regulatory filing by Musk on Tuesday.