THE HAGUE, Netherlands (AP) — One of only two known photos of painter Vincent van Gogh turns out to most likely be an image of his brother, Theo, the Van Gogh Museum announced last week.

Forensic testing by experts at Amsterdam University “confirmed the high likelihood of the boy in the photograph being the 15-year-old Theo van Gogh, and not Vincent,” the Amsterdam museum said.

The museum says there is now only one known photographic portrait of the camera-shy Dutch artist, taken when he was 19. Both photos are in the museum’s collection.

The photo now thought to be of Theo was first publicly presented in 1957 by researcher Mark Edo Tralbaut. A Dutch television program called into question whether it was indeed of Vincent in 2014, prompting further research.

“This discovery means that we have rid ourselves of an illusion while gaining a portrait of Theo,” said Van Gogh Museum Director Axel Rueger. “We have essentially returned to the situation as it always was up to the mistaken identification in 1957, with a single photographic portrait of the young, 19-year-old Vincent van Gogh.”