The 20th Anniversary of September 11

III. Response at the Pentagon

The U.S. Army Corps of Engineers (USACE) designed and built the Pentagon. Constructed in just 16 months between September 1941 and January 1943, the distinctive structure is a “unique reinforced concrete building  that … consist[s] of five concentric pentagons separated by light wells and connected by radiating spoke-like corridors.” This design would prove its worth on September 11, 2001, when terrorists crashed a hijacked airliner into the building at 9:37 AM. 

The attack destroyed a section of the outer wall and devastated the interior rings of the west side of the building. The aircraft’s fuel tanks exploded on impact, creating an intense fire and thick, black, toxic smoke that made evacuating the damaged sections of the Pentagon  very difficult. Survivors helped one another and were in some cases rescued by others who individually or in small groups went back inside to search for their colleagues. At 10:15 the outermost ring at the point of impact collapsed, but by then everyone had fled the area and no one was hurt by the structural failure. A total of 125 military and civilians died at the Pentagon, along with 59 passengers and crew on the aircraft.

Rescue workers near a collapsed section of building   Collapsed building and night in flood lights
The damage to the Pentagon shortly after the attack. 
Office of History. Photo by F.T. Eyre.
  The section of the outer Pentagon ring that collapsed
20 minutes after the crash.
 Office of History.


At the time of the attack, the Pentagon had been undergoing renovations to modernize its infrastructure and to harden the structure against bombings. Officials had long recognized the need to extensively overhaul the building, and they launched a program to do so in the early 1990s. Actual demolition and rehabilitation began in 1998 on what was called “Wedge 1”  on the west side. Based on experience with terrorist incidents overseas and in Oklahoma City in the 1980s and 1990s, the Corps’ Engineer Research and Development Center developed methods to reinforce masonry and windows against explosive blasts,  techniques that renovators applied to the Pentagon. The first tenants returned to the renovated wedge in February 2001. When the aircraft crashed into the Pentagon in September, it struck between the unrenovated Wedge 2 and the recently renovated Wedge 1.  The new force protection measures installed in Wedge 1 proved resilient, as the upgraded portion stood up extremely well to the assault. Fortunately, at the time of the attack both areas were lightly occupied by employees because of the upcoming or recently completed renovations.

Because it is a military installation, the Pentagon falls outside the purview of the Federal Emergency Management Agency. Therefore, USACE had no official response mission there as it did in New York. Nevertheless, the Director of Military Programs, Brig. Gen. Carl Strock, contacted the commander of the Military District of Washington (MDW) to assure them that USACE could assist with operations at the Pentagon if needed.  He also visited the Pentagon the night of September 11  to take a firsthand look at the devastation. As one of the only engineers on site at that time, Strock donned a fireman’s protective equipment and entered the building to conduct one of the first assessments of its structural integrity. He later described the inside of the still-burning building to a USACE historian: 

There was a lot of very acrid smoke, and a lot of water, sort of ten inches deep or thereabouts, flowing over the floors from the water cascading down from above, and very hot water coming down from above, kind of splashing all over everything.

The scene was very hard to describe, tremendous devastation. Everything was burned, blackened. Essentially, the partitions were all burned or blown away, so the [supporting concrete] columns were fairly well revealed, and we were really in there to try to assess the damage to the columns, and the structural stability of the building.

Strock also arranged for USACE structural engineers to assist with evaluating the building’s integrity and to document how it withstood the crash, both in the newly renovated space and the original.

While USACE as an agency had a limited role at the Pentagon, an active-duty engineer unit did deploy to the site. The MDW Engineer Company, based at Fort Belvoir, specializes in collapsed building rescue. These soldiers arrived in the early afternoon of September 11 and began carefully searching the affected areas of the Pentagon. During their inspection, the soldiers were  hampered by thick smoke, rubble, limited visibility, heat,  fire, and risk of  further structural collapse. After several hours, Army Engineers determined there were no additional survivors. Their mission, alongside civilian urban search and rescue teams, then became one of assisting firefighters. They also shored up the building to prevent structural failure and to protect those inside recovering victims and evidence.  Working 24 hours a day, the Army and civilian engineer teams shored up 48 columns on the first and second floors and partially buttressed 40 additional columns the week after the collapse, thus preventing additional deterioration of the crash site.

Rescue workers take a break from work   Large US flag on the side of the Pentagon building

Soldiers from the Engineer Company of the Military District of Washington take a break from shoring up the Pentagon and searching for victims. The unit was re-designated the 911th U.S. Army Technical Rescue Engineer Company on September 11, 2006. Office of History.

The large American flag placed on the Pentagon
on September 12. 
Office of History.

Army Engineers, Marines, and civilian firefighters were part of the team that hung the American flag over the impact site on September 12, moments before the arrival of President George W. Bush. The Office of the Secretary of Defense Historical Office described that moment: “For those present, the unveiling of the flag and the president's arrival helped lift morale after two days of unremitting toil and raw emotion. The picture of the huge flag flanked by firefighters and soldiers would become familiar to many, symbolizing not only the tragedy of 11 September but also the prevailing spirit of endurance and hope.”

USACE had a limited role in responding to the Pentagon attacks. However, the Corps’ most significant contribution probably took place decades earlier, when it designed and built an innovative structure of steel-reinforced-concrete that protected many of those inside and stood long enough for survivors to escape. A 2003 report by the American Society of Civil Engineers, coauthored by a USACE scientist, documented the building’s performance under attack and suggested ways to design even more resilient structures. Lessons from the September 11 Pentagon attack have since led to new and innovative measures to protect buildings and those inside.

September 2021. No. 146.

Office of History's series of articles remembering the responses to the attacks of September 11, 2001