Certainly a definitive source coming from the DoJ, but they were shut down several weeks ago, and the arrested founder apparently committed suicide 6 days ago.
This announcement is timed in conjunction with the shutdown of "Hansa Market", which has been operating as a honeypot. They shut down AlphaBay, then users and vendors migrated to Hansa, which was already honeypotted, allowing police to capture login credentials for migrating vendors.