As most of us thought that Beck is the robber, I think he was with them.
As Phsycho mentioned, Beck knows his ID by heart. However, police found the paper with his Id which suggests that Beck gave his ID to someone. But to who?
One of the employees', Miss Reiss, had been asked where she was at that time and this is what she said:
-Miss Reiss?
-I was at a club, with 2 friends, but they left at about 5AM and I left sometime later, maybe an hour, but you can ask the bartender.
This arises one thing is that she or her friends were with Beck at that time.
Furthermore, I believe that Rob was involved in there and he was the person who killed Frank because only two of them had a key to the door, isn't it?
One thing that proves Rob wasn't the only one or Beck wasn't the only one, is this line:
-Let's go, let's go! The cops are coming.
Meaning there were more that one.
One more thing, the robbery, I believe, happened between 6.30 and 7.00. One of the first lines state that the 2 security guards checked the whole bank inside once again, locked the doors, and stood outside, guarding the main entrance. But then, Rob goes for 5 minutes for "toilet" then everything happens.
One of the words that disturb me and a lot of people missed is the word "torn". The paper was torn and something was erased. If to dig more deeply, it suggests that the number 5 and number 1 were probably there to confuse us. Why something would be then erased? Something tells me that Beck was forced by robbers to give his ID. That is why at the end, when police officers asked him, he was nervous and he said he fears to be in danger