By Janat Jalil
BBC News, Washington
Officials at Dallas Zoo in the US state of Texas are trying to work out how a large male gorilla managed to escape from his enclosure.
One witness said the gorilla broke down the door in his enclosure
The gorilla, 13-year-old Jabari, fled while the zoo was full of visitors.
In a panic he attacked and injured two women and two children - zoo workers armed with tranquiliser guns pursued him but could not get a clear shot.
He was eventually shot dead by police officers, who said they had no choice as he was trying to charge at them.
"We had to shoot it," Dallas Deputy Police Chief Daniel Garcia told the Associated Press news agency.
"You can imagine the pandemonium we had out here when he got loose. We felt terrible we had to put this animal down."
Zoo officials say they are puzzled by Jabari's escape as his enclosure is surrounded by a concave wall more than four metres (13 feet) high.
One witness said the western lowland gorilla was banging on the door and broke it down.
Dallas zoo has recently had to cut staffing and maintenance because of financial problems.
However, its officials deny this had anything to do with the gorilla's escape.