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:

<div id="fModal" class="modal" >
    ...
    <div class="row-form">
        <div class="span12">
            <span class="top title">Estado</span>

          <input type="checkbox"  id="estado_cat" class="ibtn">
       </div>
    </div>             
</div>

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:

<a href='#' data-id='".$row['id_cat']."' class='editButton icon-pencil'></a>

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 it's unchecked by default)
            // and it does not work
            $("#estado_cat").prop("checked", true);
            $('#fModal').modal('show');
        });

        evt.preventDefault();
        return false;
    });
});