jquery set checkbox checked

I already tried all the possible ways, but I still didn't get it working.
I have a modal window with a checkbox I want that when the modal opens, the checkbox check or uncheck should be based on a database value. (I have that already working with others form fields.) I started trying to get it checked but it didn't work.

My html div:

    ...
    

        

            Estado

          
       
    
             

and the jquery:

$("#estado_cat").prop( "checked", true );

I also tried with attr, and others seen here in the forums, but none seem to work.
Can someone point me the right way?

EDIT:

ok, I'm really missing something here... I can check/uncheck using code if the check box is in the page, but is it's in the modal window, I can't. I tried dozens of different ways...

I have a link that's supposed to open the modal:

and jquery to "listen" the click and execute some operations like filling some text boxes with data coming from database. Everything works like I want but the problem is that I can't set checkbox checked/unchecked using code. help please!

$(function() {
 $(".editButton").click(function(){
       var id = $(this).data('id'); 

       $.ajax({
          type: "POST",
          url: "process.php",
          dataType:"json",
          data: { id: id, op: "edit" },
        }).done(function( data ) {
//the next two lines work fine, i.e., it grabs the value from database and fills the textboxes
          $("#nome_categoria").val( data['nome_categoria'] );
          $("#descricao_categoria").val( data['descricao_categoria'] );
//then I tried to set the checkbox checked (because its unchecked by default) and it does not work

           $("#estado_cat").prop("checked", true);
        $('#fModal').modal('show');
});
    evt.preventDefault();
    return false;
    });     
});