WASHINGTON (AP) - Federal authorities arrested 582 alleged gang members over a two-week period, officials said Monday, targeting an estimated 80 violent groups they say have spawned street crimes across the country. Homeland Security Secretary Michael Chertoff called the gangs "a threat to our homeland security and ... a very urgent law enforcement priority." Investigators picked up most of the offenders between July 16 and July 28 on immigration violations for being in the United States illegally. Seventy-six face criminal charges, ranging from illegal possession of a firearm to holding fraudulent documents. "For too long, these gangs have gone unchecked...