As of late May 2016, TeslaCrypt, a manufacturer of ransomware, shut down their command and control network and released the master decryption key for TeslaCrypt versions 3.0 & 4.0.

Victims of the ransomware are even being directed to a splash screen containing the master decryption key instead of instructions on how to pay the ransom to decrypt their
files.

With the release of the master decryption key, many of the existing open source decryption tools can be used to recover files affected by all versions of TeslaCrypt ransomware. Whitehats and anti-malware groups will undoubtedly be studying the release.

It is unclear at this time why the master decryption key was released.