Ive not heard of one... Though it seems abit random if they just did it for nothing...
I heard a rumor that the kidnappers were already in a shoot out with police nearby to the gas station where diaz's parents were. So it may be in the melee they decided to take hostages and therefore it was just so happenstance that Diaz's parents were in the wrong place and the wrong time.
Only thing I don't understand is how did his mom get let go? What were the circumstances?