The four members comprised a paralysed school teacher and three others.
Another family member and the ambulance driver had a miraculous escape since they had disembarked from the vehicle to push it after its engine had stopped while crossing a bridge.
The six persons were on their way to Ranchi from Chainpur of Daltongunj in Palamau district in the ambulance, Superintendent of Police Karthik S said.
The vehicle developed a snag on the Sithoya bridge over Koel river, the SP said. This happened around 2.30 am today.
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A policeman noticed the incident and immediately informed the district administration.
The district administration reached the spot within 15 minutes with a crane and ropes, and started rescue operation but all efforts went in vain, Kumar said.
While the driver of the ambulance fled from the scene, the administration managed to rescue Rakesh Rajak, the patient's son-in-law, he said.
Besides Prasad, the victims who were washed away were identified as Shanti Devi (50), Kaushal Kishore (28) and Asha Priya (18).
The driver was reluctant to pass through the overflowing bridge but agreed to drive through it on the request of the Prasad family, he said.
A massive search operation was launched to trace the victims even as rains continued.
Kumar also asked for a team of NDRF to launch a search operation.
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