1998 LaSabre won't start

Hello all..You guys usually have some good insight so here goes..My brother has a 1998 Buick LaSabre 3.8 V6 automatic.Last week he said when he went to start it it started to crank like normal and then there was a noticble clunk and everything just stopped...Any attempts to start it after that the starter motor sounded as if it was engaging but nothing happened.I went down and looked at it and thought maybe it was the starter as you could here a loud click coming from it as if the solenoid was working but had bad contacts..Took the starter off and took it to the big auto electric place near where I work and they tested it and said it was fine..Just to be on the safe side I scrounged a second starter up and had that tested and they said that too was fine..So back I go with 2 good starters but neither one would start the car...Battery was fine...I put the engine in neutral and found I could turn it with a socket and ratchet with no problem so the engine isn`t froze up..When you try to start it you can see all the accessories make a jump foward as if it`s trying to turn but the engine just won`t turn over from the starter motor(s)...Assuming the starter motors are good and the engine isn`t frozen up any ideas what to look at to make it crank over or what could be keeping it from doing so?..Thanks in advance for any ideas