How To Get The Element That Fires The Event?

I have the following html:

<div id="div1"><button id="bt1">click me</button></div>

and in script I attach events:

div1.attachEvent("onclick", handler );
bt1.attachEvent("onclick", handler );

where

function handler( e )
{ if ( !e ) e = window.event;
var elem = e.srcElement;}

So handler is attached to the div and the button. I CLICK the button. In the function handler() e.srcElement gives me the object that started the event: the button.
However, how do I get the object that fires the current event. So when the onclick is fired for the div (the onclick bubbles), what gives me the div element in this function? Because e.srcElement always gives me the button even when the onclick is fired for the div.