The 75-year-old was walking towards his vehicle with his wife when a gunman approached him and fired several shots at his back, killing him on the spot in the 1.30pm incident.
Hussain was shot in the chest and ribs while his wife Cheong Mei Kuen, 49, was injured after being hit on the hand and thigh, police said.
She is said to be out of danger and was questioned for about two hours at the Tung Shin hospital this evening.
The couple have no children of their own but Hussan has children from his earlier marriage. Cheong was said to be calm during the interview.
Closed-circuit television cameras showed the assailant was a man wearing a long sleeve checked shirt and khaki pants.
Dang Wangi police chief Assistant Commissioner Zainuddin Ahmad confirmed the incident saying it has been classified as murder.
It is believed that three suspects were involved in the incident, using a Proton Waja taxi.
An eyewitness, who declined to reveal his name, said he was at his food stall when he saw a man walking towards the couple.
"Then I heard gunshots and saw the couple crumple. I ran to safety when I saw the gunman running towards me," he said.
The shooting comes just days after crime watchdog activist R. Sri Sanjeevan was shot in the ribs in his hometown Bahau. There has also been a spate of shootings over the past month.
The AmBank Group was founded in 1975 as Arab-Malaysian Development Bank by Hussain via his holding company AIAK (Arab Investments for Asia Kuwait). He brought the bank public via the Kuala Lumpur Stock Exchange (KLSE) in 1981.
Meanwhile, Ambank group chairman Tan Sri Azman Hashim expressed his condolences to Hussain's wife and said he was sad to hear about the incident. - July 29, 2013.