Checkbox checked but not showing as checked

Checkbox checked but not showing as checked

Hi have a modal window with a checkbox that i need to check/uncheck dynamically. I use this code in the click function of a link i want to open the modal window (is that right?):

...
 $('#fModal input[type="checkbox"]').prop("checked",true); //already tried using id, class, and others... nothing works
$('#fModal').modal('show');



and if i use this

alert($('#estado_categoria').is(':checked')); 

it shows true, but the checkbox isn't checked!....
any ideas?
thanks