WASHINGTON, Sept. 10 — The government announced a heightened alert today against possible terror attacks timed to coincide with the anniversary of the Sept. 11 assault. Attorney General John Ashcroft said the latest alert, to a level higher than any declared before, was based on specific and credible information from a senior Qaeda operative who has been under interrogation. Mr. Ashcroft said President Bush had approved the change in alert status. Mr. Bush latyer said, after a visit to Afghanistan's Embassy: "We take every threat seriously. The threats that we have heard recently remind us of the pattern of threats...