Aaron Alexis, 34, was the gunman. He was a Navy veteran and native of New York. (Several items in this photo gallery have been updated and corrected.)

Sept. 16, 2013 Aaron Alexis, 34, was the gunman. He was a Navy veteran and native of New York. (Several items in this photo gallery have been updated and corrected.) Fort Worth Police Department/via AP

Cameras showed the Navy Yard gunman’s every move, but arriving police didn’t know the control room existed. Employees who had cellphones locked away to protect military secrets had no way to dial 911. Arriving police were stuck outside security gates as the base went on lockdown. Aaron Alexis came face-to-face with several Navy Yard employees who survived, including one who threw a megaphone at the gunman and ran as a shotgun blast sailed by. These are among the dozens of details in a new report obtained by The Washington Post under a Freedom of Information Act request. The report was written by D.C. police to better understand how the shooting unfolded and what lessons they could take away.

160 cameras, and no one watched.

A private security guard was in an office with monitors showing the feeds from 160 security cameras while the shootings occurred. Those cameras, police now know, covered almost every inch of Building 197 and were documenting in real time every move by gunman Aaron Alexis. But the guard locked the door, hid and notified no one that he was there with access to the information.

The lack of access to the camera feeds severely affected how hundreds of officers advanced on the shooting scene. The report by D.C. police even questions whether a D.C. officer would have been wounded by gunfire if the department had had access to the camera feeds and could have approached the suspect with better knowledge of his location.

On Sept. 16, 2013, Aaron Alexis walked into Washington, D.C.'s Navy Yard complex and opened fire. He killed 12 people and wounded four before he was fatally shot by police. Here's a timeline of how the shooting rampage unfolded that day. (Gillian Brockell/The Washington Post)

It also would have shortened by hours the work of police to dismiss conflicting reports that more than one shooter may have been in the building.

The guard is not identified in the report.

‘Full Incident Command was not clearly established.’

Part of the reason that arriving D.C. police did not know that 160 cameras existed in the building was that when police arrived, none of the six Navy Yard police officers responsible for base security were in contact with D.C. police commanders. They had gone in with guns drawn to find Alexis.

Police later learned that the naval officer at the command bus that they thought was their military liaison had no communication with the Pentagon or with the Navy Yard police force.

It is not clear whether all six of the force’s officers, including some who would have run right by the control room near the front of Building 197, knew the security monitors were there.

Too many police officers ‘self-dispatched’ to the scene.

Being in the nation’s capital, with thousands of police officers from dozens of agencies, too many with a badge also took it upon themselves to come help — or “self dispatched,” according to the report. The “sheer volume of law enforcement personnel who arrived on scene” created congestion around the perimeter of the base and in setting up a command center.

Some law enforcement agencies even brought their own cameramen: A D.C. police officer tasked with perimeter security challenged a man in plain clothes wearing a tactical vest and carrying a camera. “When confronted, he stated that he was a photographer for one of the Federal law enforcement agencies and subsequently showed his identification.”

Also, “numerous” law-enforcement agencies in Washington drove their RV-size command buses to the front gates, creating confusion about which command bus was the actual command bus. That was exacerbated by the fact that some agency leaders stayed on the buses they rode in on, separating decision makers.

The Navy Yard went on lockdown — and locked out D.C. police.

In the chaotic first minutes, a lot of police were stranded outside the Navy Yard. Gate personnel initiated a military lockdown. It had the unintended effect of locking out arriving help.

Some of the first officers on the scene had to enter the base on foot through a visitor’s entrance, as well as ask directions because gate personnel had left their posts to respond to the shooting.

D.C. police officers looked around, the report said, and decided to simply run in the opposite direction of fleeing employees.

The late order to shelter in place led to fa ce-to-face encounters with the gunman.

While officers were still scrambling to get into the building, and before the Navy sent out an alert to workers with instructions to shelter in place, some began organizing an evacuation.

One “floor captain” inside Building 197 donned his reflective vest, grabbed a megaphone and began directing colleagues to move swiftly down a stairwell, the report states. Whether the commotion attracted the gunman is unclear, but as the floor captain barked out directions, Alexis appeared a few feet away.

The floor captain “attempted to throw the megaphone at Alexis just as the gunman fired his shotgun at him. Both the megaphone and shotgun blast missed, and the floor captain and his colleagues escaped.”

Some survived, almost inexplicably. Within the first four minutes of firing his first shot, Alexis killed eight victims on the fourth floor and moved to the third floor. With what survivors later described as a “mean” facial expression, he remained silent and kept shooting indiscriminately. Then, there was this: Alexis “turns and walks away and sees a young woman hiding between a metal beam and filing cabinet. He stands directly over her and attempts to fire twice, but the shotgun does not fire. It appears that it is not loaded. He disappears down the hallway, reaching in his pocket for more shells. The woman crawls around the corner and is later evacuated by police officers.” Police didn’t know how many officers were inside the building. As arriving officers finally reached Building 197, they organized well into impromptu teams to enter the building, but if a secondary attack, or blue-on-blue shooting had occurred, the report says it would have been very difficult for police to figure out what happened. Investigators determined that 117 law enforcement officers entered the building. Eight officers from five different agencies discharged their weapons, killing Alexis.

Assault rifles are hard to use in an office building.

Although AR-15-style assault weapons have become standard weapons for law-enforcement officers when responding to an active shooter, the report by D.C. police says the large rifles were cumbersome to maneuver through cubicles and narrow hallways.

In response, D.C. police are “procuring shorter barrel rifles” for office-style environments.

D.C. police never anticipated having to police a military base.

If there are broader conclusions from the report, Police Chief Cathy L. Lanier said in an interview that one of the largest is that D.C. police leadership “never dreamed” they would have to respond to a military installation inside the city.

Although the District is home to countless federal office buildings where the department expects it would be the first to enter in an emergency, the military was assumed to be able to police its own. Neither D.C. police nor the Navy had done much planning for such a scenario.

“We’re just as guilty on our side for making the assumption that we’d never have to go in there and defend the base,” Lanier said.

Alexis left no explanation.

While the The Post and other news organizations have turned up a trail of trouble in Alexis’s past, including that he was depressed, sleepless and increasingly withdrawn, the gunman left no explanation for the rampage:

“It does not appear that Alexis gave those who knew or worked with him any prior warning or specific indication of the horrific actions he would soon carry out at the Navy Yard on the morning of September 16, 2013. . . . We will likely never know the specific reasons why Alexis would kill 12 individuals and injure several others.”

The report did not offer a single suggestion for how to prevent another mass shooting. It ends with 76 recommendations for how officers can better respond to the next one.

Clarence Williams contributed to this report.