Dec. 22, 2004 -- -- Three weeks before the deadly attack on a U.S. base in Mosul, commanders at the base had a warning that insurgents were planning a "Beirut"-type attack on U.S. forces in northern Iraq, ABC News has learned. The warning prompted them to take additional unspecified security measures on the base.
On Tuesday, 22 people -- including 13 U.S. soldiers -- were killed in an attack on the crowded mess tent at Forward Operating Base Marez in Mosul. The Pentagon said today the deadly attack was apparently the work of a suicide bomber.
"At this point, it looks like it was an improvised explosive device worn by an attacker," said Gen. Richard Myers, chairman of the Joint Chiefs of Staff, adding, "I assure you that everything possible is being done to get to the bottom of what happened and to take the appropriate steps so that we can prevent future attacks of this nature."
Earlier, sources told ABC News that investigators at the base had determined the blast was a suicide attack after finding remnants of a torso and a suicide vest that was probably a backpack.
Weeks before Tuesday's attack, soldiers from the base intercepted a document that mentioned a proposal for a massive "Beirut"-type attack on U.S. forces. The reference was apparently to the October 1983 truck bombing of U.S. military barracks in the Lebanese capital in which 241 U.S. Marines were killed.
On Nov. 27, an ABC News reporter accompanying U.S. troops during a night raid in Mosul witnessed the rounding up of hundreds of Iraqis in the densely populated Old Town district.
During the roundup, one of the suspects tossed out several crushed sheets of handwritten notes from his pocket in an obvious effort to hide them from U.S. troops. A U.S. soldier at the site, however, noticed the fallen papers, picked them up and asked an interpreter present to translate the Arabic notes.
A U.S. official at the base later described the papers as a treasure trove of information. They contained "the minutes of some type of meeting of a terrorist cell that discussed money laundering, recruitment, weapons effectiveness and future operations," the official said. One of the possible future operations was described as a "Beirut"-type attack, and the notes referred to the importance of seeking and supplying information about Iraqis working for the U.S. military.
Following the discovery of the papers, commanders at the base -- which is about three miles south of Mosul and is used by both U.S. troops and the interim Iraqi National Guard forces -- ratcheted up already tight security.
The likelihood that Tuesday's attack on Forward Operating Base Marez was an "inside job" first emerged with an online message allegedly posted by a radical Sunni Muslim group, the Ansar al-Sunnah Army, which claimed responsibility for the blast and called it a "martyrdom operation," a reference to a suicide attack.
Earlier today, a new message, allegedly by Ansar al-Sunnah, said the suicide bomber was a 24-year-old man from Mosul who worked at the base for two months and had provided information about the base to the group.
The message also said the suicide bomber used plastic explosives hidden inside his clothes and that the "martyr" had gotten married about a month ago. It also claimed the group would post a video of the attack on the Web. The authenticity of the message, however, has not been verified.
The bombing at the mess tent was one of the deadliest attacks against U.S. troops in Iraq since the start of the war. Early reports indicated the massive explosion might have been the result of a rocket attack.
The attack has led to questions about security at the facility in Mosul, which has seen an increase in insurgent attacks since the U.S. military assault on the city of Fallujah last month.
U.S. military officials say there were plans to build a bunker-like mess hall at the base. Dining halls at bases have been the target of mortar attacks across central Iraq in the past.
A day after the attack, a group of U.S. soldiers wounded in insurgent attacks arrived at the Ramstein Air Base in Germany for treatment at a military hospital.
A C-140 transport plane carrying some 50 patients -- most of them injured in Tuesday's attack -- touched down at the U.S. base in Germany. A spokeswoman for the Landstuhl Regional Medical told The Associated Press that the medical facility was expecting at least eight critical-care patients in the group.
Tuesday's attack claimed the lives of 13 U.S. service members, five U.S. civilian contractors, three Iraqi National Guard members and one unidentified non-U.S. citizen, according to U.S. military officials. Of the 69 people wounded, 44 were U.S. soldiers.
In Mosul today, U.S. tanks and armored vehicles swept through the streets as a curfew was imposed on Iraq's third-largest city. The governor of Mosul banned the use of all five bridges into the city, and said anyone breaking the order would be shot, according to Reuters.
Once a relatively peaceful city of Kurdish and Arab residents, Mosul was ranked as the U.S. military's greatest success story in the early days of the Iraq occupation. But just days after a massive U.S. operation in Fallujah began last month, insurgents carried out a number of deadly attacks on police stations across the city.
U.S. military officials believe many insurgents hiding in Fallujah before the assault -- including Jordanian militant Abu Musab al-Zarqawi -- may have fled to Mosul.