US troops clashed with Iraqi insurgents in the former rebel stronghold of Fallujah on Thursday residents waited to re-enter the city for the first time since a November offensive to crush the militants. American warplanes and tanks shelled parts of the city, hampering the return of about 200 families. Iraq's interim government had expected closer to 2,000 families to return Thursday, despite the city's devastated infrastructure. More than 200,000 Fallujah residents fled ahead of the offensive. They have been living in tent cities erected in nearby towns and villages for the last seven weeks. Three Marines were killed today in fighting in al-Anbar province, which includes Fallujah. The military did not say where they were killed, but the deaths were announced as clashes took place in Fallujah. And the military is continuing an investigation into how a suspected suicide bomber was able to penetrate a US military base near Mosul Tuesday, killing 22 people.