As soon as he had shot dead his first victims at 7.15am — and two hours before the massacre of 30 more at Virginia Tech — Cho Seung Hui calmly ran a diabolical errand.

Leaving the bodies of Emily Hilscher and Ryan Clark, he returned to his room a short walk away. Once there he stared into a camcorder to record the last of the QuickTime video clips that the world would watch in stunned horror 48 hours later, and downloaded all 27 files — ten minutes of enraged and expletive-filled rantings against the rich and powerful — on to a DVD.

“When the time came, I did it. I had to,” he said, in what police believe was his final message. In another, recorded shortly before the massacre, he talks in a low voice, unlike the mostly angry rants that dominate the videos: “This is it. This is where it all ends. End of the road. What a life it was. Some life.”

He then packaged the DVD with a 23-page, 1,800-word manifesto of hate and grievances. The carefully bound document also contained 43 self-taken photographs, 11 of them showing Cho brandishing in different poses the Glock 9mm pistol and Walther P22 handgun that he was about to use to commit America’s worst single gun massacre. As police rushed to the first shooting, Cho, 23, walked a quarter-mile, through Virginia Tech’s main gates and two further blocks, to 118 North Main Street, Blacksburg’s tiny post office.

It was busy. Monday, April 16 was tax-filing deadline day, and the post office was filled with people sending in their last-minute returns. Cho paid $14.40 (£7.20) for a large, overnight Express Mail envelope into which he placed the package that he had spent days — perhaps six, officials believe — assembling.

The female clerk was friendly and helpful, unaware that she was holding a DVD showing Cho staring into a camera, in a number of different settings — in a car, in his dormitory — ranting. “Do you know what it feels like to be spit on your face and to have trash shoved down your throat? Do you know what it feels like to have throat slit from ear to ear? Do you know what it feels like to be humiliated and impaled upon a cross?

“You had a hundred billion chances and ways to have avoided today. But you decided to spill my blood.”

He twice glorifies, calling them martyrs, “Eric and Dylan”, a reference to Eric Harris and Dylan Klebold, the teenagers who carried out the Columbine High School massacre eight years ago today.

“You sadistic snobs,” he continues in a dead monotone. “Happy to be nothing but a piece of dogs***. You have vandalised my heart, raped my soul and tortured my conscience. You thought it was one pathetic boy’s life you were extinguishing. Thanks to you, I die. Like Jesus Christ, to inspire generations of the weak and the defenceless people.”

At one point he appears to be reading from a script. At another he is leaning forward to switch the camera off, confirming that he was acting alone.

In one photograph, Cho has carefully laid out on a table, in a mosaic, 50 rounds of hollow-tipped ammunition. But in the post office he made some bizarre errors. He sent, but never explains why in his videos, the package to NBC headquarters in New York. But he got the address wrong.

Instead of the correct 30 Rockefeller Plaza, he wrote “30 Rockefeller Ave”. He also got the postal code wrong. Ever helpful, the clerk reduced it from 6 digits — all US postcodes contain five — but it is still not NBC’s code.

The clerk placed a US Postal Service time stamp on the envelope. It was 9.01am.

Just over half an hour later, Cho entered the Norris Hall classroom building where he killed another 30 students and lecturers, before turning his Glock on himself.

Late on Tuesday night, the package arrived in New York, but was not delivered to NBC until 11am. An alert mail clerk noticed the Blacksburg return address, and the sender’s name: “A. Ishmael”, eerily similar to the words “Ishmael Ax” scrawled in red ink on Cho’s arm when his body was found.

An NBC security officer, Brian Patton, donned rubber gloves to open the package. Copies were made, and the originals handed over to the FBI at noon. At 4.30pm, the first images and video clips were broadcast.

Karan Grewall, Cho’s roommate, said he had never seen him with guns, or the knife and hammer he poses with in other photographs. “It was a totally different person. He was staring straight at the camera, and he never stared into our eyes or even looked at us.”

From scene to screen

April 16, 7.10am Cho Seung Hui shoots the students Emily Hilscher and Ryan Clark dead on the fourth floor of West Ambler Johnston Hall

7.15 911 call logged to Virginia Tech police department. Cho returns to Harper Hall to record final video

7.30 Officers begin following leads about a “person of interest”. Police decide not to close the campus

9.01 Having walked to Blacksburg post office, Cho mails a package to NBC containing video clips and writings

9.26 Students are alerted to situation after authorities send an e-mail entitled “Shooting on Campus”

9.40 Armed with two pistols and multiple clips of ammunition, Cho walks into Norris Hall. He fires dozens of rounds at professors and students: 30 die

9.45 Second emergency call. Police find front doors chained from inside. They break in, and follow sound of shots to second floor. By the time they arrive Cho has killed himself April 18, 11am Package arrives in NBC building. Staff are suspicious and open it using rubber gloves

Noon Copies are made, and contents delivered to the FBI

4:30pm First video clips broadcast. Package contained 23-page written statement, 28 video clips and 43 photos

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