The Herald revealed yesterday that the chief suspect in the murder is a 16-year-old boy who had not previously come to garda attention but another teenager who was arrested is considered a "very dangerous indeed".
A source explained: "He is very dangerous -- he was one of a mob who inflicted horrendous injuries on two American tourists as they walked in the Wellington Quay area last April.
"The victims in this case had their faces ripped open by broken bottles -- it was a savage unprovoked attack."
While this teenager is not suspected of carrying out the fatal knife attack on Mr Heinrich, it is believed that he was at the scene in Rialto and was "heavily involved" in the first altercation that occurred minutes before the shocking murder.
Mr Heinrich was knifed several times in the stomach and sustained a severed artery after he and his friend Robert Rinker became involved in an argument with two youths.
Three teenagers have now been arrested in connection with the killing, as well as one teen's mother, but all had been released without charge by yesterday afternoon.
The German friends were living at the Herberton apartment building while they studied at Dublin's Griffith College. They had planned to return home for Christmas.
The two young men had been standing on the balcony of their first-floor apartment at St Anthony's Road after a night out, shortly before tragedy struck. Words were exchanged with locals who left the scene and returned with knives.
During the attack Mr Rinker was stabbed in the hands and also received a cut to the side of his head. He was taken to the intensive care unit at St James's Hospital but his condition has since improved.
Gardai arrested a 16-year-old local youth for questioning at lunchtime on Saturday, while a 19-year-old later walked into a station and was detained.
The mother of one of the youths was arrested on Sunday afternoon for allegedly withholding information about the stabbing.
All three were later released without charge and gardai at Kilmainham are currently preparing a file for the Director of Public Prosecutions.
The third teenager was arrested on Sunday night and taken to Pearse Street station for questioning.