I don't know about it being the tallest building -- 30 stories
From ABCNews.com:
A small plane crashed into a skyscraper in downtown Milan today, setting several floors of the 30-story building on fire.
The plane crashed into the 25th floor of the Pirelli building in downtown Milan. The weather was clear at the time of the crash. Smoke poured from the opening as police and ambulances rushed to the area.
The president of the Italian Senate, Marcello Pera, told Italian television it "very probably" appeared to be a terrorist attack but soon afterwards his spokesman said it was probably an
accident. A transport official told Reuters the plane had reported problems with its undercarriage and was circling the city ahead of trying to land at a local airport.