Crankshaft pulley bolt broke off in shaft. Shop says treads in crank damaged.

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My power steering went out and battery light came on while driving last week. I pulled over and shut engine down. Opened hood and saw belts loose or off. Drove approx. 1/4 apparently they had removed this bolt then and claimed it was already broken but had enough treads left to reuse along with some lock tite. After pulling in I turned engine off and told mechanic what had just happened. Little while later he started my car and drove it under a bay to look at problem then restarted with second guy under hood. A piece of metal flew out and hit the guy so they shut it down and told me what happened. The next day I was told they found the broken off crank bolt under the vehicle and that the treads in crankshaft had been damaged beyond repair. They think I will need a new crankshaft or rebuilt engine. They are saying no liability on their parts since bolt had been broken two years earlier when they replaced a timing gear that had broken. I believe they did not properly inform me then or now and they should assume part or all the liability to repair my vehicle.