I think the relevant part is here, with some details later:

As it turned out, the answers she sought may have been closer than she imagined: Richardson’s stepbrother, Darius Dawson, is the man who set the car on fire with his body in the trunk, officials said. Chicago Police Department arrest records indicate that CPD officers wanted to charge Dawson with first-degree murder in Richardson’s death. But prosecutors wouldn’t sign off on the case.

Basically dude drove around with the body in the trunk then set it on fire.

It’s always possible in situations like these that they could prove he set the car on fire with his dead body inside (hence concealment of murder) but maybe couldn’t definitively prove he was actually the one that killed him. Better to get him in jail on something easily proved than risk him walking on charges with incomplete evidence.

Looks like this guy is doing a good job keeping himself in prison though.