Hiding from gunmen in an office building, a group of people heard gunshots getting louder and the clatter of a weapon being run along the metal banister of a staircase, according to a survivor of an Islamic extremist attack in Kenya last week. Then a gunman opened the door of the room where the workers huddled, said he was with al-Shabaab and began shooting.In a higher floor of the Nairobi building, witnesses said, a gunman pushed against the door of a bathroom where women were hiding and said: "We know you're in there. Open."New details of the January 15 assault on the dusitD2 hotel and office complex in Kenya's capital emerged on Tuesday during a church memorial for six employees of digital payments company Cellulant who were killed. The tribute touched on the terrifying last moments of the young men, as well as the anguish of colleagues in safety, who were receiving phone messages from trapped friends."Our teams would tell us, 'Help us, help us. This is where we are. The gunshots are coming closer and closer,'" said Ken Njoroge, CEO and co-founder of Cellulant, which operates in several African countries.Workers in the Cellulant offices on the fifth and sixth floors of the Cavendish building, beside the hotel, thought a blast at around 15:00 on the day of the attack was a "gas explosion" and weren't particularly alarmed, Njoroge said. A few minutes later, they heard another explosion followed by gunfire. They headed for the exits."The teams began to mobilize," Njoroge told hundreds of mourners. "I think it became very clear that all was not well."Some 83 out of 100 workers in the Cellulant offices at the time escaped, but the rest didn't make it out and split into two groups, he said. Six hid in a small room under the stairs on the first floor, and the other 11 returned to the fifth floor, where they also split: The men into the men's bathroom, and the women into the women's bathroom.Njoroge, who was on a business trip to Zambia at the time and quickly returned to Kenya, compiled the account from colleagues. He said people couldn't tell where the shooting was coming from and that one colleague said: "'When you hear the gunshots, all of your other senses go away. You can't see, you can't hear. Nothing.'"Kenyan security forces have been praised for their quick response to the attack, in contrast to their fumbling response to the 2013 attack on the nearby Westgate mall that killed 67 people. The attack last week killed a total of 21 people, including a police officer, and was declared over nearly 20 hours after it started. More than 700 people were evacuated. All five attackers, among them a suicide bomber, died. A number of suspected accomplices are under arrest.Al-Shabaab, which is linked to al-Qaeda, claimed responsibility. The group says it attacks Kenya for joining an African Union force fighting against it in its home base of neighboring Somalia. The extremists have been heavily targeted by US air strikes.Continuing with his account, Njoroge said one or two gunmen flung open the door to the men's bathroom on the fifth floor, grabbed 36-year-old Ashford Kuria Maina, pulled him out and killed him."'They've taken Ashford and we've heard some gunshots,'" a colleague texted to friends who were safe, providing a nearly instantaneous report on the horror.Attackers pushed the door to the women's bathroom but were unable to enter because the women pushed back, according to the account. The gunmen left.For the six Cellulant employees hiding on the first floor, it was "as if this guy was actually walking on their heads" as they listened to a gunman descend the stairs, according to Njoroge.The gunman "opened the door and had a conversation with them, introduced himself as al-Shabaab" and killed five people, the company CEO said. The sixth employee survived because her male colleagues protected her by tucking her at the back of the room. She smeared blood on herself to pretend that she was also a casualty, Njoroge said.The five men who died there were Denis Munene Mwaniki, 29; Jeremiah Mathai Mbaria, 31; John Wanyaga Ndiritu, 29; Kelvin Kariuki Gitonga, 28; and 23-year-old Wilfred Kareithi Waihura, who was supposed to be at a first aid training with colleagues elsewhere but came into the office for a 15:00 conference call with a customer.A week before the al-Shabaab assault, victim Ashford Kura Maina talked with a colleague about what they would do if they were under attack, said associate Alex Kimani. He said his longtime friend "kept giving options."Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook.