A painting at London's Tate Modern by famed Russian-American artist Mark Rothko has been defaced.

On Sunday a spokesperson told Reuters that authorities were investigating the incident in which a visitor came into the museum and defaced one of Rothko's seagram murals. "Tate can confirm that ... there was an incident at Tate Modern in which a visitor defaced one of Rothko's Seagram murals by applying a small area of black paint with a brush to the painting," the Tate said in a statement according to Reuters.

The incident which took place in the afternoon is a shocking one for the museum. Rothko's art is worth millions of dollars. Reuters reported that his work sells at auction for tens of millions. The BBC reported that the painting, Black on Maroon from his Seagram murals was the one defaced.

In the 1950s Rothko was commissioned by the Four Seasons restaurant in New York to create the Seagram murals that hang in the Tate currently. CNN reported that Rothko, after being commissioned by the Four Seasons, decided that the restaurant was not the appropriate place for his artwork.

No one has been arrested as of yet, but the BBC reported that Vladimir Umanets, the founder of a movement he has dubbed Yellowism, claims responsibility. He said to the BBC, "Art allows us to take what someone's done and put a new message on it."

CNN reported that Tim Wright who was visiting the museum at the time saw the vandalism take place. He said that him and his girlfriend were at the Tate when a man walked into the exhibit. He thought nothing of it until a smashing sound was heard. "It was very surreal. It wasn't something we expected to see. One minute he sat down, and the next minute he put his foot over the barrier," Wright said to CNN.

After the incident CNN noted that the gallery was closed briefly because of the vandalism. Mark Rothko's children Kate Rothko Prizel and Christopher Rothko said to CNN, "The Rothko family is greatly troubled by yesterday's occurrence but has full confidence that the Tate Gallery will do all in its power to remedy the situation.

Our father donated his legendary Seagram paintings to the museum in 1969 sensing the commitment of the institution to his work and impressed by the warm embrace it had received from the British public. We are heartened to have felt that embrace again in the outpouring of distress and support that we and our father have received both directly and in public forums."