A man has been killed and a woman critically wounded when police say the man tried to stop another man from beating a woman in a crowded San Antonio parking lot.

The incident happened Friday afternoon outside of a Wal-Mart in northwestern San Antonio.

Police Chief William McManus says a man was assaulting a female companion when the good Samaritan drove up and demanded that he stop. McManus says the assailant drew a handgun and fired at the man, killing him and critically wounding a female bystander. A woman in the vehicle with the good Samaritan was injured by shattered glass.

McManus says the beaten woman escaped and a police helicopter followed the suspect's vehicle for 10 miles before his arrest.

No identities have been released.