AJAX :: Set The Visibility My UpdatePanel To True Upon A Button Click Event

I'm attempting to set the visibility my UpdatePanel to true upon a button click event (as shown below). It should be rather straightforward but unfortunately it's not working at all. Any idea why it's going wrong?

//Markup
<asp:UpdatePanel ID="UpdatePanelSearchSubject" runat="server" UpdateMode="Always" Visible="false">
<ContentTemplate>
<p>
</p>
<p>
<asp:TextBox ID="findSubject" runat="server" Width="248px"></asp:TextBox>
</p>
<asp:RadioButton ID="peopleSearch" runat="server" Checked="True"
Text="People" GroupName="searchSubject" />
<asp:RadioButton ID="groupSearch" runat="server" Text="Group"
GroupName="searchSubject" />
<br />
<asp:Button ID="btnGetGroups" runat="server" Text="Search"
OnClick="btnGetGroups_Click" BackColor="#CCFFCC" />
<br />
<br />
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="btnSearchSubject" />
</Triggers>
</asp:UpdatePanel>
//code behind
protected void btnSearchSubject_Click(object sender, EventArgs e)
{
UpdatePanelSearchSubject.Visible = true;
}