Aaron Hernandez's Brother Arrested After Allegedly Planning School Shootings

Dennis "D.J." Hernandez -- the older brother of Aaron Hernandez -- has been arrested again ... this time after cops say there was concern he had been planning school shootings at UConn and Brown University.

The disturbing allegations are all spelled out in new Bristol Police Department documents, obtained by TMZ Sports ... where cops say D.J. showed signs this month of being "gravely disabled and a danger to society."

According to the BPD, several people came forward in early July claiming D.J. had been acting erratically ... and sending alarming text messages, including one that allegedly read, "we're taking lives if s*** isn't paid up. It's been years in planning just taking notes, names and locations. They talked their way into this and it's almost point game."

Cops wrote in the docs that they were told by a woman claiming to be D.J.'s ex that instead of using her car to go to his scheduled July 7 Court date ... he actually drove to Brown Univ and UConn and "went into a number of classrooms and buildings." Cops say another person told them they believed D.J. had traveled to the campuses in order to plan school shootings.

Police say additionally, D.J. had posted messages with threatening language on his social media page on July 19 ... including one that they said read, "Will I kill? Absolutely, I've warned my enemies so pay up front."

According to the docs, police showed up at a residence D.J. was at later that day ... and a scary police standoff ensued.

Cops say they tried to get D.J. out of the home ... but he had told them that he was "on foot, armed and that if we approached him, he would kill us all." A short time later, D.J. exited the residence ... but according to cops, he "began advancing towards the police with his hands outstretched to his side."

"He began yelling shoot me," police added, "and disregarded the numerous police commands."

D.J. was then tasered and fell to the ground ... and moments later, cops took him into custody -- before they transported him to a local hospital for evaluation.

While there, police say he continued to make threats at the hospital ... saying he would kill "anyone who profited off of his brother Aaron." Dennis, cops said, "mentioned specifically ESPN, but not any people."

D.J. was ultimately released from the hospital and taken into the Bristol Police Department, where he was booked on charges of threatening and breach of peace. He was held on a $250,000 bond.

He's now due in court for a hearing on the matter next week.