How the Dallas massacre unfolded

The short hallway inside El Centro Community College is left in shambles. Ceiling tiles and office walls are mangled. Bullet holes piercing through barriers and doors still visible.

Large pieces of hallway walls have been cut out and removed by FBI investigators, including the area near the school’s library, where Dallas police say killer Micah Johnson wrote the initials “R.B.” in his own blood.

It’s the hallway where SWAT officers engaged in a violent shootout with Johnson then used a robot carrying explosives to end the standoff.

“All the gory stuff is gone,” El Centro Community College Police Chief Joseph Hannigan said as he escorted reporters on the same path Johnson followed on July 7, the night of the deadly shootings that killed five police officers and wounded seven other officers in Dallas. Two civilians were also injured.

“My personal belief was that he was just going to wing it. I don’t think he had any initial plans to enter El Centro, but I don’t think he anticipated the response that he got. So he had to adjust on the run,” Hannigan said.

For nearly two weeks, El Centro Community College has been a crime scene as FBI investigators worked to collect evidence from inside the school and biohazard teams cleaned up the bloody remnants of the standoff.

With that work now done, the school’s police chief Tuesday walked reporters through how the massacre unfolded.

The scene inside that hallway still captures the horrific intensity of that night.

“He was brazen enough to just hide behind the walls and open fire,” Hannigan said.

Hundreds of gunshots

In the final moments, only about a 30-foot long hallway separated SWAT members and the killer. Hannigan said it appears at least 200 gunshots were exchanged between SWAT officers and the gunman in this area alone during the standoff that lasted several hours.

The chunk of the wall where Johnson wrote the “R.B.” on the wall is gone, taken by the FBI.

Johnson was cornered in an area behind the school’s library. SWAT teams had blocked the only two ways out of the space where Johnson was barricaded.

SWAT team negotiators tried to communicate with Johnson. Dallas Police Chief David Brown has described the killer as delusional yet in control.

“He, basically, just lied to us, playing games, laughing at us, singing, asking how many (officers) did he get and that he wanted to kill some more,” Brown told CNN the weekend after the shooting.

As negotiators talked to Johnson, tactical teams were preparing to launch a robot equipped with a pound of C-4 explosive down that short hallway.

Hannigan said Johnson saw the robot coming and desperately fired at the robot, trying to disable it before the explosive detonated. The robot didn’t stop, and police detonated the bomb, killing Johnson on the spot.

Path of a killer

The carnage started outside on the east side of the community college building around 9 p.m. As Black Lives Matters protesters marched past the school in downtown Dallas, Johnson drove up in a dark SUV. He got out and started a conversation with two Dallas police officers before pulling his weapons and shooting them.

Other officers quickly turned toward the shooting scene and exchanged gunfire with Johnson, Hannigan said. Johnson tried to shoot his way into the building through glass doors on the east side but officers inside the school fired back at the killer.

“We believe at that point the return fire was so much, he shot out all the Lamar street doors in an effort to escape. But the return gunfire was so much that he started moving down towards the corner,” Hannigan said.

The next killing was caught on camera from the roof of a hotel across the street. The video shows Johnson gun down an officer who approached and was using a building pillar as a shield.

Johnson then turned the corner to reach the north side of the building where he shot his way through glass door entrances and immediately found a staircase and went to the second floor.

This is where officers picked up a blood trail and followed the killer’s path.

Johnson walked through the school’s library. It’s a surreal scene inside the building, because everything has been left intact since the day of the shooting. Johnson walked right past a table holding an unfinished 1,500-piece puzzle of The Beatles rock band.

Inside the school, there were students on the seventh and eighth floors as well as students trapped on the main floor and the basement.

On the north side of the building, Johnson now had the upper hand, shooting through windows down at officers on the street.

“They were hiding behind their police cars, and he was shooting at anybody that moved,” Hannigan said.

But it’s also the spot where Johnson eventually found himself cornered and trapped. Once officers identified where the shots were coming from, tactical teams moved into place and kept him from escaping.

The El Centro campus will reopen to staff on Wednesday. Classes resume Thursday.

CNN’s Jason Morris contributed to this report.