Billionaire entrepreneur Elon Musk, who is attempting to walk away from his deal to acquire Twitter Inc., is seeking documents from advertising technology firms as part of his quest to gain more information on bot and spam accounts on Twitter, according to filings in a Delaware court on Thursday.

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. The trial is scheduled for Oct. 17.

Musk's lawyers have subpoenaed both Integral Ad Science (IAS) and DoubleVerify for any documents or communications on their involvement to review accounts or participate in any audit process 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 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.