Tuesday's attack was the deadliest car bombing since March 13, when a parked car exploded near a bridge in Tahrir Square, killing 18 people. Nobody has claimed responsibility.
Lieutenant-Colonel Steven Stover said the command believed the attack was carried out by a "special group" led by Haydar Mehdi Khadum al-Fawadi, who has been sought by US and Iraqi forces for months.
The US uses the terms "special groups" to describe splinter factions of al-Mahdi Army, which is led by Muqtada al-Sadr, a Shia religious leader.
Washington has repeatedly accused al-Mahdi Army of receiving weaponry and assistance from Iran, Iraq's eastern neighbour.
"We believe [al-Fwadi] ordered the attack to incite [Shia] violence against Sunnis; that his intent was to disrupt Sunni resettlement in Hurriya in order to maintain extortion of real-estate rental income to support his nefarious activities," Stover said.
In other news, dozens of Shia militia members surrendered to Iraqi forces on Wednesday in Maysan, hours before a deadline set by Nuri al-Maliki, the Iraqi prime minister, for them to lay down their arms.
Officials said the four-day deadline given to the fighters in the southern province was successful although some fighters had escaped before a crackdown set to begin at midnight (21:00 GMT).
"The deadline has been very successful. We have received many weapons, especially today," Abdel Qader Jassem Mohammed, the Iraqi defence minister, told the AFP news agency.
Mohammed said some militia members had run away before the start of the operation but that "many" others were still in Amara, the capital of Maysan.
Focus on Amara
Amara is said to be a stronghold of al-Mahdi Army and a centre for weapons smuggling from Iran.
The clampdown in Amara follows similar efforts in Basra and Baghdad's Sadr City district.
Brigadier-General Saad al-Harbiyah, police chief of Amara, said 60 militia members had surrendered before the offensive and that hundreds of landmines were also discovered on Wednesday.
Earlier, al-Maliki told security forces to refrain from random arrests of al-Sadr supporters during the planned operation against militia groups.
A statement from al-Maliki's office said: "He has stressed that only outlaws must be arrested and he hopes that the Sadr leaders will help in isolating such elements to get rid of them."