referencing dynamically created controls

I have web form, which has some of its content created dynamically. I need to access these items when a button is clicked (in the onclick event). So far I have drawn a blank.
Here is the code for the creation (in pre_render):

// Selected col
CheckBox cb = new CheckBox();
cb.ID = "cbxSub" + _Ids.Keys[ i ];
cb.Text = "";
TableCell cSub = new TableCell();
cSub.Controls.Add( cb );
row.Cells.Add( cSub );

And teh code to get the state of the control (onclick):


foreach ( Control ctrl in Page.Controls)
{
if (ctrl is CheckBox)
if ((ctrl as CheckBox).Checked)
if ((ctrl.ID != null) && (ctrl.ID.Length > 6) && (ctrl.ID.Substring( 0, 6 ) == "cbxSub"))
Session[ ctrl.ID ] = "true";
}

What am I doing wrong?