A wave of bombings targeting Shiites in Iraq killed 72 people on Thursday, deepening sectarian tensions that exploded just after the last American troops left the country in mid-December.

The bombings began early in the morning when explosions struck two Shiite neighborhoods in Baghdad, killing at least 27 people. A few hours later, a suicide attack hit Shiite pilgrims heading to the holy Shiite city of Karbala, killing 45, said provincial official Quosay al-Abadi. The explosions took place near Nasiriyah, about 200 miles (320 kilometers) southeast of Baghdad. Hospital officials confirmed the causalities.

The attacks began in Baghdad with the explosion of a bomb attached to a motorcycle near a bus stop where day laborers gather to look for work in the Shiite Sadr City neighborhood. One of those who witnessed the attack said it filled the area with thick black smoke.