More details from IDF:
Sources in the IDF say this was a well planned attack. The terrorists in the training camp were training on how to plant explosives, fire qassam rockets and RPGs. They also trained on how to sneak into Jewish towns and their training was done by high Hamas officials.
More details in post 25 and also Maariv quotes the IDF as saying the field at that "community center" where Hamas and other terrorists were training (and those are all the casualties) was named the Ahmed Yassin Field. They were also training on how to hijack vehicles.