CALCUTTA, India?A video camera atop the U.S. cultural center in Calcutta captured the faces of gunmen on motorcycles who killed four police guards and wounded 20 other people, police said Thursday.

“The video films have given us vital clues for investigation,” Deputy Commissioner of Police Saumen Mitra said.

Videotape from the security camera on the roof of the American Center gave a frame-by-frame account of Tuesday’s assault, he said.

“After questioning several eyewitnesses and seeing these video films, our artists have drawn sketches of the two attackers,” Mitra said.

Calcutta police have picked up more than 70 people for questioning, but most were allowed to go free after questioning, Mitra said. Six people, including three Bangladeshis, have been arrested in connection with the attack, but none of them are believed to be the gunmen.