People look at the damage to a bridge after a suicide bombing in Ramadi, west of Baghdad. REUTERS/Osama Al-dulaimi

The Islamic State planned a coordinated attack in at least two sections of Baghdad Thursday in an attempt to free more than a thousand prisoners, according to the Institute for the Study of War (ISW). The militant group fired mortars, used at least one car bomb and sent in several suicide bombers in an effort to infiltrate a prison in Baghdad and demolish the office of Badr Organization, the group coordinating with the U.S. in anti-ISIS attacks.

Reuters said the death toll is at least 16, but Iraqi media claimed the number is much higher. Dozens were wounded in the attacks.

This is the first “complex” and “penetrating” attack the militant group formerly known as the Islamic State in Iraq and Syria (ISIS) has carried out in Iraq’s capital since it declared its caliphate and seized Mosul in June, ISW said.

Fourteen mortar rounds fired at the Adala prison were likely shot from Taji, a Sunni district in northern Baghdad where the militants have a significant stronghold, according to Ahmed Ali at ISW. One of the mortars landed in a nearby restaurant, killing four people. A car bomb near the prison killed three and wounded 10. At least two men wearing suicide vests also intended to detonate, but were arrested.

Many ISIS-affiliated Twitter accounts shared the news, claiming the militants had freed more than 1,500 prisoners. The accounts also helped to circulate a YouTube video that militants claimed showed the bombing. The Ministry of Justice denied reports of a prison break, according to Iraqi News.

“The attack also signifies that, despite the heightened defenses of Baghdad in the aftermath of the fall of Mosul, ISIS is still able to carry out attacks in an area where it is unlikely to have active sleeper cells,” Ali wrote.

It's unlikely that the Sunni militant group has sleeper cells in the Shia-dominated area surrounding Adala.

Over the course of the summer, though, there have been several warnings of possible sleeper cell attacks in Baghdad. One report was called a “Zero Hour” attack and was discussed in early July. At the time, Iraqi security officials said that there were at least 1,500 members of a sleeper cell in Baghdad who were prepared to infiltrate the U.S.-run Green Zone at any time.

The attempted infiltration came just days after President Obama announced an expansion to the U.S. air campaign in Iraq. Part of the strategy was to begin striking ISIS targets from the air near Baghdad, in an effort to help Iraqi forces regain control of the area. The decision to increase strikes meant that the U.S. had changed its original mandate of aiding the humanitarian crisis currently facing Iraq’s Yazidi minority population.