Seems I cannot check a checkbox in jQuery

[Krokonoster]

Nope, but now I'm not so sure what is really my problem:
Look at this code:

if (jQuery.inArray(id, currentSelection) > -1) {
    // just to make sure $(this) is the checkbox
    alert($(this).attr("checked") + "-" + $(this).attr("type") + "-" + $(this).attr("name") + "-" + $(this).attr("id"));
    $(this).attr("checked", "checked");
    alert($(this).attr("checked"));
    //$(this).prop("checked", true);
}

Both those alerts (before and after I check it) say "checked" for "$(this).attr("checked")". But on the view (it's a modal by the way) nothing is checked.....