According to the latest information release, the shooter had attached a trigger device to his weapons which allows for rapid firing similar to  a weapon that had been  modified to fire full auto.
There are a number of such devices on the market which attach to the weapons trigger 
or  there is a reciprocating rifle butt stock  device which works in a similar way.  These items are legal and work very effectively and allow rapid firing equal to a modified or legal full auto weapon.
Videos of such devices can be viewed on YouTube.  You will be amazed at how well they work.