The federal investigation into the Chicago Police Department might not have happened without help from a confidential source within Chicago’s law enforcement community.

Craig Futterman is a clinical law professor at the University of Chicago. He initially learned about the shooting of Laquan McDonald from a police blotter.

“Like all too many people, my eyes glazed over because it was something that I saw all the time,” he said.

But later, a confidential source tipped him off to the existence of a dashcam video showing the fatal shooting of McDonald by police.

“They told me it looked like nothing short of an execution,” he said.

The source told Futterman they were worried the case could get buried like other police related shooting cases. Futterman worked with reporters Jamie Kalven and Brandon Smith. By court order, the city eventually released the dashcam video showing the night Laquan McDonald was shot 16 times by Officer Jason Van Dyke, who now faces a first degree murder charge.

Futterman said anyone in law enforcement that speaks up puts themselves at great risk.

“You become a pariah. You’re blackballed. The job can be a dangerous job. You call for back-up. You’re in a sticky situation. Back-up may be slow to arrive,” he said.

Futterman said a code of silence existed in the Chicago Police Department long before Mayor Rahm Emanuel took office, before former Police Superintendent Garry McCarthy was appointed. Futterman said most people in law enforcement do a good job, but even the most upstanding officers can be bound by the code of silence.

He recalled a time when some of his law students were at a public housing residence and observed special operations officers hit a man with their car. He said the driving officer got out, beat the man and planted drugs on him in front of residents and other police officers who worked in that community.

Futterman said he saw the community officers get into an argument with the other cops.

“But they both also told me they would never testify, that they would never report this. And they hated it at least as much as I did because it put their lives in jeopardy. It created hostility in the same place they had to work everyday,” he said.

Futterman said the source brought the Laquan McDonald video to light because they were fed up. He said the source wasn’t expecting it to result in a federal investigation into the department or for other videos showing other police incidents to be released.

Susie An is a WBEZ reporter. Follow her @soosieon.