The Javanese account
A local account of the disaster was captured in the Javanese book Panjeblugipun Redi Kelut (The Eruption of Mount Kelut) by Dayawiyata and Yudakusuma. It was written 40 days after the event. I read it in 2018.
The book on Panjeblugipun Redi Kelut. Cover by BPNB Yogyakarta.
According to the local account, at 1:30 AM, or around one and a half hours after the explosion, the boiling mudflow had engulfed Blitar. It was estimated that the speed of the mudflow reached 65 km/hour.
The deadly mudflow occurred because the crater of Mount Kelut contained a lake with an estimated 40,000,000 cubic meters of water at the time.