Investigators say the victim was delivering the last of his packages and letters along E 65th Street Friday when someone walked up to his mail truck.
"He made a statement that was kind of a non-verbal statement to get the carrier's attention, and without warning punched the carrier three times in the face," said U.S. Postal Inspector Jeremy Leder.
So far, it doesn't look like any mail was stolen.
"At this time there's nothing we found that was stolen from the truck, no personal items, no mail," Leder said.
The victim gave enough description for detectives to compile a sketch of the suspect -- a sketch that hit home for a man who lives on the same block as the attack.
"I might have seen him before walking up 64th," said Jim Conway. "So apparently I think he's from around here."
The victim doesn't believe his attacker was under the influence or suffering from mental illness, saying he was gone as quickly as he came.
"The suspect basically ceased his attack and then just strolled down the street," Leder said.
U.S. postal inspectors are now offering up to $5,000 reward for information about the attacker. He's believed to be in his mid-30s and is 5 feet 8 inches tall with brown hair and has a cut under his left eye.
Investigators consider this man very dangerous, saying attacks on letter carriers are rare.
"And I think the reason why is that people know there are steep penalties for anybody that assaults a federal employee," Leder said.
Because the attack was on a postal worker, whoever is responsible could case federal felony charges and up to eight years in prison. By contrast, typical state charges would only rise to a misdemeanor and less than a year in jail.