"I don't know why. I couldn't just help myself."

Mark shrugged at the police officer as he said those words. He couldn't believe them himself, but tried to sell it to the cop as the reason for stabbing the man, kid really, on the train ride home.

"Take me through it again," the officer said. They stood on the sidewalk next to the steps that led to the El platform that Mark took into work every day.

Mark recounted the story one more time. He had boarded a block from his office in the Loop. The train was empty, which was odd for a Wednesday at 5:30 p.m. But it was Sept. 12, 2001, and most of the country was still in shock at what had happened the day before. Few offices were open that day. Mark himself hadn't wanted to come in. Katrina's uncle worked in one of the towers in New York City and they hadn't heard whether he had gotten out before it fell. They both went to bed the night before with an empty, raw feeling in their stomachs, as if they hadn't eaten for days.

Mark awoke early and decided to go to the office. The train into town was empty. The Loop looked as deserted as it did on Sunday mornings. Mark went through the motions at work — looking over copy, writing headlines, formatting photos for the website that employed him. He could tell from hourly traffic reports that nobody cared what he was posting. If it didn't report on news from New York or Washington, D.C., it didn't matter.

He checked in with Katrina twice. No word from her dad about her uncle. The gnawing pit in Mark's stomach grew, even though he ate two sandwiches for lunch. He promptly left at 5:15 and caught the 5:30 brown line to Ravenswood.

"The train was empty," Mark began. "I sat next to the window and saw three of the kids get on at a later stop. They started hassling an old lady who had gotten on at the stop after mine. I recognized her from other days and knew she had a hard time walking, which is why she sat next to the door. Those kids didn't like that."

Mark saw the kids taunt the lady. "They figured they had the train to themselves and they outnumbered her and me," he said. The lady stood to move, and as she passed one of the boys blocking the aisle, he pushed her.

"That's when you got up," the officer said.

"Right," Mark replied. "They didn't need to pick on her. They could sit anywhere."

"What did you say?"

"Help her up!" Mark shouted at the boys. "Help her up and let her sit where she wants. Leave her alone."

"Ooh, a tough guy, eh?" one of the kids said. "Mind your own business or I'll make you mind it."

Mark saw the glint of something flashy in the kid's hand and shook his head.

"You know how to use that thing? Cause I sure as hell do."

"Did you?" the officer asked.

"I taught self defense classes in college and studied martial arts. I could handle myself," Mark said.

It had been several years, and he was rusty, but the skills and muscle memory - like riding a bike - never went away.

The boy lunged at Mark, who moved to the side, grabbed the arm that held the knife and twisted it until the kid dropped it.

He had disarmed the kid when one of the other two decided to attack Mark.

"That's when you turned and stabbed him," the officer said.

Mark nodded. "I didn't mean to, he just stepped to me and I moved my arm."

Mark mimicked the motion for the officer, who took notes and nodded.

"Why did you get involved in the first place?"

"They pissed me off," Mark told him. "I'm in a bad place right now."

"Aren't we all?" the officer said.

The cop put away his notebook. "I checked with the EMT. They say the kid's gonna be alright. Really more a scratch than a knife wound. To me it was self defense, but I'll have to make a report. I'll call you if we need anything further."

Mark shook the officer's hand and walked home, still wondering about Katrina's uncle.