3:59 PM PT -- Victim Giovanni Arnold's attorney, Daniel Szalkiewicz, tells us, "Our office was deeply upset to learn that the NYPD has decided not to prosecute Cardi B, Offset, or any of their body guards following their actions last year after the Met Gala."

He continues, "Both my office and my client cooperated with the NYPD at every opportunity but had not heard from them since the weeks immediately following the incident ... We hoped that those responsible for Mr. Arnold’s injuries would be charged accordingly. It appears that was wishful thinking."

"What the NYPD did today was endorse violence. They have given Cardi B, Offset, and all celebrities a license to assault people who say things they don’t like; and that is a dangerous precedent to set."

The security team for Cardi B and Migos is off the hook in an alleged assault on an autograph seeker at last year's Met Gala.

Law enforcement sources tell TMZ ... the rappers' security guards won't face any charges, and the assault case has been closed. We're told the alleged victim refused to be interviewed by NYPD detectives. That's despite the fact he was clearly seen on video catching a beatdown.

TMZ broke the story ... Cardi, Offset and Quavo were all leaving a Met Gala party when the melee began. The rappers were accused of ordering their security team to put hands on an overzealous fan.

None of the security guards was ever arrested, and it was only CB and the Migos rappers who were facing potential criminal charges -- but now, everybody's walking away scot-free.

There's still a civil lawsuit, though. The fan's lawyer, Daniel Szalkiewicz, filed last May claiming he suffered injuries to his face, neck, back and body.