LOS ANGELES — Federal and local investigators were trying to determine whether an arsonist set a fire that turned an unfinished downtown apartment tower into a block of flames so hot they melted freeway signs and cracked windows in office high-rises as far as a block away.

More than 250 firefighters battled flames that engulfed the wooden frame of the seven-story Da Vinci complex around 1:20 a.m. Monday. It took hours to douse the flames and left an unsafe, smoldering heap of wood and metal. Heavy equipment tore into the remaining scaffolding Monday night.

No one was injured.

GAITHERSBURG, Md. — A mother and her two young sons, one of them just a month old, were killed Monday morning when a private jet crashed into their two-story wood-frame home, which was engulfed in a fireball immediately, authorities and witnesses said. All three people on board the plane also died.The jet's fuselage crashed into the front lawn of an adjacent home, which was heavily damaged by fire, and investigators believe one of its wings, which had fuel inside, sheared off and tore through the front of the Gemmell home, said Robert Sumwalt, a National Transportation Safety Board member.