Flex :: Ctrl And Space Key Used In AS Class?

I have a switch statment in my action script class. I have a function onKeyDown (event: KeyboardEvent) which contain a switch statement.which control the different keys. But now I want to handle the same functionality which TAB key is doing using two keys CTRL and SPACE key. I want add a case more in my switch statement first I pressed CTRL and then SPACE key then a a specific funtion is called. How two keys used in my switch statement.......

private function onKeyDown(event: KeyboardEvent) : void{
if (popUp.displayPopUp){
switch (event.keyCode){
case Keyboard.UP:
case Keyboard.DOWN:
[Code] .....