The BBC has asked a US court to compel a California-based website to produce records which could identify who leaked footage of the next "Doctor Who" series.
A 53-second clip featuring the new Doctor Jodie Whittaker and two still images of the actor were shared on Tapatalk a week ago. BBC Studios said it takes "theft and illegal distribution of our content extremely seriously".
The BBC has filed an application with the California Federal Court, reports bbc.com.
A spokesman added that BBC Studios "will strive to protect our programme-makers, audiences and licence fee payers from any breaches of security - ensuring 'Doctor Who' fans enjoy the final and fully completed version of the episode when it premieres".
BBC Studios began an investigation as soon as it was made aware of the unauthorised leak.
It is not the first time that "Doctor Who" material has appeared online earlier than planned. In 2014, BBC Worldwide apologised after scripts and footage from the series were accidentally placed on a publicly accessible server.
And in 2005, an individual who allegedly leaked the first episode of the revived drama was sacked following an investigation.