NEW YORK — Americans are marking 24 years since the Sept. 11, 2001, attacks with solemn ceremonies, volunteer work and other tributes honoring the victims. Many loved ones of the nearly 3,000 people killed were joining dignitaries and politicians at commemorations Thursday in New York, at the Pentagon and in Shanksville, Pennsylvania. On Thursday morning, Denise Matuza, Jennifer Nilsen and Michelle Pizzo boarded a bus from Staten Island for Lower Manhattan — each wearing T-shirts emblazoned