add custom web user control in onclick event handler function

I have an asp.net project and I have a panel in it in which I want to add multiple user control on the panel by clicking on the button, It does this when clicking on the button

System.Web.UI.Control _msg = (System.Web.UI.Control)Page.LoadControl("msgsend.ascx");
pnl.control.add(_msg)

but it clears all the user controls on the panel before and adds only one user control. What should I do to avoid this?