NEW YORK (CBSNewYork) – A teen accused of beating an off-duty firefighter last weekend on the Upper East Side has been arrested.

Police said Damir Johnson, 17, was among the six teens initially allegedly to have punched 38-year-old John Mongielo last Saturday morning on East 86^th Street.

The off-duty firefighter told the teens to stop harassing an elderly couple, but wound up with a concussion and broken teeth, police said.

Johnson was arrested Thursday and charged with assault. Police said at this time he is the only suspect.

"@FDNY firefighter who acted as a good samaritan, continues to recover from serious injuries. We thank him for his actions & wish him a swift recovery," NYPD Chief of Detectives Dermot Shea tweeted early Thursday.

The search continues for the other suspects – two boys and three girls between the ages of 15 and 17.