The police don't need to determine who specifically killed Jake, only who participated in the crime.
If Bob and his six friends work together to kill Jake, then all seven of them are guilty of murder, even it was only Bob who actually killed Jake. For example, if two of Bob's friends hold Jake so he can't move, and the other four of Bob's friends distract the other people in the room so they can't see what's happening, and then Bob stabs Jake with a knife killing him, then under US law they're all considered guilty of murder.
Now in a trial, the prosecutor would have to prove beyond a reasonable doubt that each of these seven people is guilty of murder. However, in order to do the prosecutor doesn't need to prove what exactly each of the seven people actually did, only that each person participated in the crime. In my example above, it doesn't make a difference who stabbed Jake or who held him while he was being stabbed, or who tried to prevent anyone from witnessing Jake being killed, anyone who did any of these things is guilty of the same crime. The prosecutor just needs to prove that each defendant did any one of these things, not that each defendant did one particular thing.
So while the police don't need to know who actually killed Jake, but they do need to who participated in the crime. If say, Bob and his six friends are all known gang members, while the rest of people in the room have no known criminal associations it may be easy for the police to identify them as suspects and then build a case against them. It's possible that they'll only be able to find evidence that shows that some of the seven participated the crime, but not others. It's even possible for the actual killer to be acquitted while the rest are convicted.
Potentially a lot of outcomes are possible, but the police don't need to identify the killer specifically.