An Upper East Side building worker was killed this morning after tumbling three stories, authorities said.

“It sounded like a gun shot,” said nearby handyman Amerigo Sarno, 44, of the painter’s fall from the window at about 11:20 a.m. near the corner of Park Avenue and East 76th Street.

“He wasn’t moving [and] I thought he was dead,”

Sarno, who gave an umbrella and poncho to the coworkers of the fall victim to keep him dry, added that the man started moving after a few moments.

The man, whose identity was not immediately released, was rushed by EMS to New York Hospital where he died, officials said.

“His head was bleeding but he was conscious,” said massage therapist Anthony Tripi, 46, who was visiting a doctor’s office when the accident occurred.

“It looked pretty gruesome,” Tripi added.