According to the prosecution:
AH summoned the two accomplices (I am assuming they are cooperating with the prosecution at this point) and they drove to Dorchester to pick up the victim at his house. After leaving in AH's car, the victim texted his sister saying "did you see who I am with", when she replied who?, the victim texted back "NFL" (aka Aaron Hernandez) and then texted "just so you know" (so clearly he knew something was up). Then they drove to the industrial park where AH shot him 3 times as he got out of the car. Then AH walked right up to the victim and stood directly over him and shot him twice more in the chest at point blank range. AH and the two other men then left the victim dead and drove back to AH's house...where they have security video of AH walking in the house with the gun (I guess he didn't do a good enough job destroying his security system).
Sounds like they have both witnesses and plenty of other evidence on this. Obviously AH is innocent until proven guilty (and he certaintly has the best lawyers money can buy). But it sounds like they have plenty of evidence to prove this was pre-meditated and plenty of evidence to prove it was AH.