modalPopupExtender is not modal

 I have created a popup using Ajax ((ver 19.1.0.0) modalPopupExtender in a form in a c# aspx project in VS 2013 with the following parameters in the form.aspx viewed as markup:

%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %> <script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.2/jquery.min.js"></script>

.modalBackground
        {
            background-color:Gray;
            filter:alpha(opacity=70);
            opacity:0.7;
        }
    .modalPopup  
        {  
            background-color: #FFFFFF;  
            border-width: 3px;  
            border-style: none;  
            border-color: black;  
            padding-top: 0px;  
            padding-left: 0px;  
            width: 350px;  
            height: 150px;  
        } 
		

<cc1:ModalPopupExtender ID="mp1" runat="server" PopupControlID="Panl1" TargetControlID="cmdTest"  
    CancelControlID="Button2" BackgroundCssClass="modalBackground">  </cc1:ModalPopupExtender><asp:Panel ID="Panl1" runat="server" CssClass="modalPopup" align="center" style = "display:none">  <iframe style=" width: 350px; height: 150px; background-color:white;" id="irm1" src="NoLicense.aspx" runat="server" ></iframe>  <br/>  <asp:Button ID="Button2" runat="server" Text="Close" />  </asp:Panel>  

The popup form is NoLicense.aspx. The popup appears OK when called from code via mp1.show() but execution continues - the popup form is not modal. How can I make it modal?