From Police Heroes, a book by author Chuck Whitlock:

Kenneth Tietjen, thirty-one, was at the 33rd Street PATH station on September 11. When he got word of the attack, he commandeered a taxi, then got into an emergency vehicle and went to the World Trade Center. He evacuated people, some of whom were badly burned, from the North Tower. He then took the last respirator before running into the South Tower which collapsed moments later.

Tietjen volunteered for the Belford Engine Company when he was eighteen. A Port Authority police officer for nine years, he received a commendation for bravery in 2001 when he tackled a man who’d stabbed the sergeant he was working with. In 1996, he received a special commendation when he subdued a man who’d stabbed a police officer after ramming a patrol car in the Holland Tunnel. He had almost completed his training to become a member of the ESU.

Portraits of Grief, The New York Times

He Claimed the Respirator

As a boy, the two things that scared Ken Tietjen most were fire trucks and police cars. So he took some ribbing from his family when, as an adult, he chose a job that required him to ride in both.

Mr. Tietjen, a Port Authority police officer, was at the 33rd Street PATH station when he heard about the terrorist attack, said Laurie Tietjen, his sister. Mr. Tietjen commandeered a taxi, banished the driver to the backseat, and drove to ground zero. He rushed into the north tower and helped people down, but when he emerged to get a new respirator, only one remained, his partner recalled.

Smiling, Mr. Tietjen said, “Seniority rules,” took the respirator and ran into the south tower. Moments later, the building fell.

Typical, said Ms. Tietjen, noting his commendations for bravery this year, received for tackling a man who had stabbed the sergeant he worked with. As a firefighter several years back, he returned to a burning building to rescue an unconscious colleague.

One of those Mr. Tietjen rescued on Sept. 11 attended his memorial Mass. But he did not stay because he became overwhelmed.

Ms. Tietjen said: “My brother had a choice whether to go back and he chose to go back in. I wouldn’t expect anything less from him.”