SAN JOSE, Calif. (KRON) – We continue to bring you the latest in the deadly VTA mass shooting in San Jose.

Now, the Santa Clara County District Attorney’s office says this may have been avoided if federal authorities had informed the San Jose Police Department about the shooter’s troubling past.

“First of all, there are many mechanisms by which we share information between local law enforcement and our federal partners. We have telephones. We have emails. We have relationships with one another,” District Attorney Jeff Rosen said. 

None of those resources were used by the Department of Homeland Security to inform Santa Clara County Law Enforcement that Samuel Cassidy, the VTA shooter, was detained by the feds back in 2016. 

Santa Clara County District Attorney Jeff Rosen says it would have been useful to at least been contacted about the basics.

“Why was Mr. Cassidy stopped? What was the reason for that? What was found? Why didn’t you share that with local police,” Rosen said.

What concerns the D.A. most is what was reportedly in Cassidy’s possession when he was detained.

“I am concerned about an individual who has books about terrorism and is so angry at their coworkers that they are writing it down, not typing it but taking pen-to-paper and writing down how angry they are. Now that’s not a crime to do those things but it is certainly something for a D.A., for a police chief, would be of interest,” Rosen said. 

This information comes to light in the wake of learning that law enforcement found an arsenal of weapons and ammunition in Cassidy’s home. 

Weapons he may not have been legally able to obtain if San Jose police were given a heads-up from the feds about the 2016 incident.

“One thing I can say is had certain information been shared with local law enforcement there would have been a response. The police would have conducted some kind of investigation, talked to some neighbors or coworkers, would have talked to Mr. Cassidy. What would that have led to? We’ll never know? But there are interventions and tools police could have used,” Rosen said. 

He says like a referral for mental health treatment or workplace mediation or asking a judge for a gun violence restraining order. 

However, 5-years later 9 Valley Transportation Authority employees are gunned down. Samuel Cassidy takes his own life. The district attorney is taking action to prevent another tragedy.

“The reason that I am asking these questions and seeking answers and information is not about blame because the ultimate responsibility for the mass shooting is Sam Cassidy’s. What I am doing is trying to prevent the next Sam Cassidy from committing another mass murder,” Rosen said.