An intern working for Sen. Barack Obama's presidential campaign was arrested Friday at the Des Moines airport after authorities flagged an outstanding arrest warrant for aggravated battery in Illinois. Derrick Lamon Johnson, 20, of Glen Carbon, Ill., was also charged with fleeing from justice.Tommy Vietor, Iowa press secretary for the Obama campaign, said Johnson had been working for the campaign for about a month as an unpaid intern. Vietor said Secret Service agents learned of the arrest warrant during a criminal background check after Johnson was scheduled to drive in a campaign motorcade. Johnson was booked into the Polk County Jail.