Where did the writers of the gospels get their information?

Matthew and John were both eyewitnesses to the majority of the events related in their accounts since they were a part of the 12 apostles (Matthew was not witness to birth or earlier accounts, but no doubt was told of these things by Mary or Jesus Himself; John 21:24).  Mark is said to have gotten his information from Peter, but we have no real proof other than questionable tradition.  Luke was a physician and historian and he got his information by questioning people who were close to Jesus while He was alive and wrote down all of the testimony (Luke 1:1-4).  The Holy Spirit was also at work helping these men write down the information accurately, making it inspired scripture (2 Peter 1:21).