How can I change the state of the itemRenderer of one member of a flex 4 spark data group?

Question

I have a "step indicator" made out of a DataGroup:

<s:DataGroup id="stepNumbers" itemRenderer="stepNumberRenderer" horizontalCenter="0">
<s:layout>
<s:HorizontalLayout verticalAlign="middle"/>
</s:layout>
</s:DataGroup>

where the dataprovider is 1,2,3,4,5.

The item renderer has 2 states, "normal" and "active". Within Actionscript, I’d like to be able to set each number to "active" in as needed. So, I need to set "2" to its active state, i.e. stepNumbers[1]. What's the syntax for this?