At Least 143 Die in Attacks At Two Sacred Sites in Iraq
Suicide bombers and other attackers detonate mortars, grenades and roadside bombs among crowds of Shiite Muslims gathered for holy day observances at mosques in Baghdad and Karbala, killing at least 143 people; dead reportedly include pilgrims from Iran; death toll is highest in Iraq since United States-led invasion; much of angry reaction to attacks is focused on occupation; American officials suspect Abu Mussab al-Zarqawi, Sunni militant who wrote to senior Al Qaeda leaders seeking help in starting 'sectarian war' by attacking Shiites to spur counterattack by them on Sunnis; one of four suicide bombers is arrested when his explosive-laden vest fails to detonate; US intelligence official sees no link between attacks and attack on Shiites marking same holy day, Ashura, in Quetta, Pakistan; maps; chronology of attacks against Muslim targets; photos (M)