1996 A6 2.5 TDi AEL engine, 170k miles

My garage called to tell me my timing belt had broken when they started my car to move it. I've just opened the cover to find the camshaft is broken in two!

The car was in the garage because it stopped a couple of days before. The garage reported the injection pump belt was missing teeth and replaced it (only fitted 12k miles before).

They got it running roughly prior to static timing being set up.

I'm trying to figure out what happened. Did the belt go because of repeated starting attempts by the garage with incorrect timing? The belt was 25k miles old and not near the end of its life. Once the belt went, did the camshaft go from the valves forced up by the pistons?

I'm lost how to proceed. Is the garage responsible, or were they just unlucky? At the back of my mind is a infrequent but recurring fault "00550 - Start of Injection Regulation
17-00 - Control Difference" but this this might be a red herring?

Any ideas, please?

The TDI motor is an interference design, meaning both the pistons and valves occupy the same face (at different times). Possible explanations I can think of:

Tensioner bad (or set improperly)?

Or... could be bad timing-- if the flywheel moved when they reinstalled the belt & did not notice it.

They probably didn't rotate the crank by hand to verify there was no banging...

If could be luck... (my turbo went last week, after getting a 4-wheel alignment)... was there any evidence of valve or piston damage (ie-- pull the heads)

you fitted the pump belt 12k before and now its broken again??was the cam belt replaced before??