Enter event issue in textbox because of update panel inside updatepanel

Hi Friends,

I have some issue in my code.

This is search functionality code. When I enter some text in textbox and click search, the results are displaying correctly. But when I enter text in textbox and click enter without pressing search button , all results are displaying irrespective of text which I entered. Please let me know any issues with the code.

This is the sample code. Based on the below sample code when I enter text in textbox and enter, it is always firing "Letter_Click" event. not hidden search button click. I dont know why this issue is happenning. Is it because of update panel or I am not sure.Please can you help me out from this issue.

<!--Update Progress is now out of the update panel, otherwise we were getting some JScript errors--> <div class="updateProgress"> <asp:UpdateProgress ID="updateProgress" AssociatedUpdatePanelID="UpdatePanel1" DynamicLayout="false" DisplayAfter="0" runat="server">    <ProgressTemplate>         <asp:Image ID="Image3" ImageUrl="~/Images/spinner_small.gif" runat="server" AlternateText="" GenerateEmptyAlternateText="true" />     </ProgressTemplate> </asp:UpdateProgress> </div>    <asp:UpdatePanel ID="UpdatePanel1" runat="server" Visible="true" >            <ContentTemplate>        

                         <asp:Panel ID="paraPanel" Visible="false" runat="server" Style="cursor: move;background-color:white;border:solid 1px Gray;color:blue">               </asp:Panel>         <div id="pcontent">       <div class="messagepanel">        <mes:Message ID="userMessage" runat="server"/>     </div>                 <asp:UpdatePanel ID="upLetters" runat="server"  UpdateMode = "Conditional" Visible="true" RenderMode="Inline">            <ContentTemplate>      <ul class="searchbar">          <li class="firstitem"><asp:ImageButton id="btn_" runat="server" CommandArgument="_" OnClick="Letter_Click" ImageUrl="~/images/popup/hash.gif" AlternateText="Search #" Width="19" Height="18" TabIndex="-1" CausesValidation="false" /></li>          <li><asp:ImageButton id="btnA" runat="server" CommandArgument="A" OnClick="Letter_Click" ImageUrl="~/images/popup/A.gif" AlternateText="Search A" Width="19" Height="18" TabIndex="-1" CausesValidation="false" /></li>            </ContentTemplate>    </asp:UpdatePanel>

</div> <!--pcontent-->

    <div class="searchtxtbox" runat=server id="searchtxtbox">         <asp:label id="lblCriteriaCaption" AssociatedControlID="txtSearchCriteria" runat="server">Missing Title</asp:label>         <asp:TextBox ID="txtSearchCriteria" runat="server" TabIndex="1"></asp:TextBox>    </div>       <div class="searchbtn">         <asp:ImageButton id="btnSearch" OnClick="btnSearch_Click"  runat="server" ImageUrl="~/images/search_btn.gif" AlternateText="Search" CausesValidation="false"/>         <asp:ImageButton id="btnHiddenSearch" OnClick="btnHiddenSearch_Click"  runat="server" ImageUrl="~/images/search_btn.gif" AlternateText="Search hidden" CausesValidation="false" style="display:none"/>         </div>

    </ContentTemplate>    </asp:UpdatePanel>

Thanks Bhavani