Two Spies in Big Trouble

"Run, Tom! Run!" cried Robbie. That was the moment when the two boys knew they had just made the biggest mistake of their lives. The reason why it was a mistake will take some explaining. Six months before this, Tom and Robbie were just playing a game of "spies" in the woods behind their houses. They heard shots. And then two police officers came out of nowhere. Then there were more shots, and the police officers went to investigate. They did not come back. Then more police officers came. They went out into the woods to investigate. They did not come back. What were the shots? What happened to the police officers? And what happened six whole months later? What was the biggest mistake of Tom and Robbie's lives?