In the latest development of the lawsuit against Elon Musk's refusal to buy Twitter, the tech billionaire has opted to seek documents from advertising technology companies for more information on Twitter's bots and spam accounts. 

Twitter has sued the Tesla chief executive, who has accused Twitter of hiding information about how it calculates the percentage of bots on the service, for attempting to walk away from the $44 billion agreement. A trial is scheduled for October 17.

According to recent filings in a Delaware court,  Musk's lawyers have subpoenaed both Integral Ad Science (IAS) and DoubleVerify for any documents or communications on their involvement in reviewing accounts or participation in any audit of Twitter's user base.

IAS and DoubleVerify, which are both based in New York, use technology to independently verify that digital ads are viewed by real people. Advertisers use the services to ensure the ads they pay for are seen by potential customers and not automated bots.

Twitter, IAS and DoubleVerify did not immediately respond to requests for comment.

In response to a tweet by a user who questioned how Twitter audits its service and also linked to a Reuters story on Musk targeting the ad firms, Musk tweeted:  "Those are the questions that Twitter is doing everything possible to avoid answering".  

In a countersuit earlier this month, Musk claimed that Twitter's monetizable daily active users are 65 million lower than what the company has touted. Twitter has said it stands by its disclosures.

The metric measures users who log onto Twitter through the website or apps that are able to serve ads or used paid products like subscriptions, according to Twitter filings.