I heard a faint boom in the distance. I finished what I was writing and was just about to check if it was anything significant when the first reports came through of a bomb blast at the Islamic University on the other side of Islamabad.
The suicide bomber blew a hole in the wall at Islamabad's Islamic University [AFP]
This was followed shortly by reports of a second explosion in the university's café.
We headed out to the scene. Traffic was understandably heavy and our progress was blocked. So with my cameraman, Salahuddin, we convinced a passing motorbike to take us to the scene, waving our press passes as we passed the police checkpoints.
We arrived and walked into the building, stepping over a trail of blood spots leading from the front door, down the stairs and off into the near distance. This was where the first explosion went off. The air hung heavy with the smell of smoke, every step was punctuated with the crunch of broken glass.
Pools of blood
Making our way upstairs, it became clear where the suicide bomber had ended his deadly mission. Thick pools of dark red blood marked the area and a hole had been blown in the office wall.
School notes lay smoldering; pieces of clothing lay strewn across the ground while sandals sat abandoned in the middle of the carnage.
Police moved in and started clearing the building, ushering people slowly and gently away from the seat of the blast.
Outside, I saw a slim young man, his shirt soaked in blood, small pieces of skin clinging to him. Shaukat Ahmed Mir spoke calmly and with a composure that belied the horror he'd just witnessed.
He had been standing with friends outside the building when the first blast shook the ground. As glass showered down around him, he bravely ran in through the door to do what he could to help.
"I ran into the building. The people were in a lot of pain, crying and shouting. I started dragging people out who were suffering, they were suffering a lot. Other people inside had been separated by the blast. There were body parts lying everywhere. I helped to collect them to take them to the ambulance."
Then, across the campus there was the second blast. It shook the ground as it exploded in the University cafeteria. Shezad Haider had been praying. He told me "It felt like an earthquake. I thought the roof had caved in above me. It was terrifying".
The wounded were taken to local hospitals. Some were in a serious condition, but for others there was simply nothing the doctors could do.
As police carried out their investigations inside, students hung around outside waiting for news. Many just kept ringing numbers on their mobiles trying to contact those they thought might have been caught in the explosions.
Pakistan's Interior Minister, Rehman Malik appeared, his convoy sweeping in through the main gates. As he mounted the steps, it began as a small chant but was quickly taken up by others. "Shame, shame, shame".
He thought it was something else, a shout of support and clenched his fists together and signalled to the crowd. He walked smartly inside, but stayed just two minutes before he was out and heading back to his car.
The crowd began again with "Shame", then a few shouted "Death to Rehman Malik", again picked up by the majority. Some students grabbed handfuls of dirt and threw it at his convoy, expressing their anger that they had been the victims of such a bloody attack.
Akbar Ali Bhatti told me: "The government's security forces have failed. The suicide bombers hit this place where they teach the Quran and good things. The bombers didn't come from the sky, they passed all the checkpoints".
People hung around for a long time, reluctant to leave but knowing there was little more they could do.
Islamabad's International Islamic University has more than 12000 students, more than half of them women. People come from all over the world to study here. Established in the 1980s, it is growing a big reputation.
Late on Tuesday, the Taliban issued a statement insisting it was not responsible for the attack – pointing the finger instead at the Pakistani security forces, claiming this was an operation to discredit them.
In recent attacks the Taliban have been quick to claim responsibility. But this attack is a reminder that while Pakistan's army wages war in South Waziristan, nowhere in the country is safe.