Back then, Pettis was a coach for the Chicago White Sox, who had arrived in town only a few hours earlier for a scheduled game that night against the New York Yankees.
"You could smell the smoke. It wasn't a good feeling that day," Pettis said Wednesday before a home game against the Pittsburgh Pirates. "It's so sad that so many people lost their lives, and it's ruined other peoples' lives. ... It's like it was a movie, it's like that wasn't something that actually happened. I still can't believe it."
What he does believe is the importance for Major League Baseball — and all Americans — to take a moment to remember Sept. 11.
Players, coaches and umpires wore American flag patches embroidered on the side of their caps in commemoration of the tragedy. Special lineup cards were used, and patriotic on-field tributes were planned for the day's 15 games, involving all 30 teams. Flags were half-staff, and there were moments of silence across baseball.
There were impromptu remembrances, too.
In New York, art students made a chalk drawing in blue and orange on the sidewalk outside Citi Field, showing the Twin Towers, the Mets logo and the words "Never Forget."
New York Mets manager Terry Collins wore an NYPD hat and his players wore caps representing other first responders during batting practice before hosting Washington.
"You'll always remember how you felt on 9-11," Collins said.
With so many tributes planned at the stadium, a memo was written on a board in the Nationals' clubhouse — "Note: Everyone on the field @ 6:55."
Both dugouts were filled with applauding players, managers and coaches as members of rescue and security organizations marched onto the field. The Mets and Nationals then lined up along the baselines for a moment of silence and the national anthem.
At Rangers Ballpark in Texas, the 531st U.S. Air Force Quintet performed the national anthem instrumentally. The honorary first pitch was thrown out by former prisoner of war Jessica Lynch, who was 19 when she was captured along with five other soldiers after the U.S. Army's 507th Maintenance Company took a wrong turn and came under attack in Iraq in 2003. She was held for nine days before being rescued.
The Cleveland Police Department presented the colours at Progressive Field before the national anthem at the Indians' game against Kansas City.
Cleveland's Jason Giambi was with Oakland when the Athletics were in the 2001 playoffs against the Yankees. He recalled the atmosphere at Yankee Stadium being "unbelievable," even more electric than usual for the post-season.
"It will always be a time I'll remember, going out there playing against the Yankees during that time," Giambi said. "It kind of healed the nation, especially the city of New York, which was hit so hard. There they were, the Yankees playing in the playoffs, going all the way to the World Series."
Giambi signed with the Yankees after that, and spent seven seasons in New York.
"Playing there all those years, the kids, the firefighters, the people who lost their lives saving the other lives, I'll always remember that, very much so," Giambi said.
At Cincinnati's Great American Ball Park, where the Reds hosted the Chicago Cubs, a steel beam from the World Trade Center was on display courtesy of the Cincinnati Fire Museum.
Before San Francisco hosted Colorado at AT&T Park, first pitches were thrown out by two San Francisco firefighters who went to New York in the days after Sept. 11 to provide help and support. Dean Crispen, captain of Station 28, and Derek O'Leary, driver of rescue squad one from Station 1, flew on the first commercial flight allowed to land in New York.
Pettis and the White Sox had arrived in New York 12 years ago around 2-3 a.m., and he was awoken by a phone call from a friend checking to make sure he was OK.
"I said, 'Yeah, I'm OK, I'm asleep.' He said, "you don't know, do you?" Pettis recalled. "I turn on the TV and I see that the building — smoke's coming out of the building — and they said there had been a plane crash."
Like so many others, Pettis thought maybe it was just a tragic accident before the second plane hit the other tower.
The White Sox were staying in a hotel at Grand Central Station, a little more than three miles from the World Trade Center site. Pettis and the rest of the staff worked to locate everybody with the team, and to get out of the building, with concerns about more potential attacks.
"We were going down the stairs and you hear this rumble, and we're going what the heck is that?" Pettis said. "We just kind of take off running out the doors, and now we see people running out of the train station, and we had no idea what they were running from."
Pettis can't believe it's been 12 years. Before going to the ballpark on Wednesday morning, he turned on his TV knowing what he was going to see.
"It took me a minute to get up and get my day going because I started watching some of the stories and listening to some of the people talk about being there, and then seeing some of the messages that were left for families," he said.
Pirates infielder Clint Barmes remembers exactly where he was and what he was doing 12 years ago. He was only 22 years old in his second season of pro ball, and on the way home after winning the championship with high-A Salem the night before.
"I didn't get a chance to see anything on TV until I got home later that evening. ... Had my car already packed ready to go," Barmes said. "I woke up, jumped in my car and started driving home before I realized exactly what happened.
"There's a lot of things that goes through your mind when something like that happens. It was a scary moment for sure."
To veteran Tampa Bay manager Joe Maddon, it was a day to remember the terrible images on television, and a pal.
"One of my best friends in college has just been appointed the head of the N.Y. Port Authority. Neil Levin," he said. "So then I'm thinking, 'OK, Neil's pretty cool, he's the boss, he's going to show up late, he's not going to be there early."
"As it turns out he was having breakfast that morning in that restaurant on the top floor. So we lost Neil on that one," he said. "So whenever I hear 9-11, this date ... while I was riding my bike today, seeing the flag at half-mast, I thought of Neil."
Washington star Bryce Harper was just 8 and at home in Las Vegas when the attacks occurred.
"I was in my mom's bed, watching TV. I used to watch 'CHiPs' and 'Saved by the Bell' in those days. Then it came on, all over the news," he said Wednesday. "I was trying to understand it, we were trying to decide whether I should go to school."
"I remember my dad came right home from work. I remember he came in the door and I ran right to him, gave him a big hug and told him, 'I love you.'"
Harper said he and some Washington teammates hoped to visit the National Sept. 11 Memorial plaza in lower Manhattan late Wednesday night, after their game against the Mets, to see the "Tribute in Light."
"We wanted to see the beams," Harper said. "I think it's important."
AP Baseball Writer Ben Walker and AP freelancers Tommy Magelssen in Arlington, Texas, Mark Schmetzer in Cincinnati, Steve Herrick in Cleveland, Rick Eymer in San Francisco and Mark Didtler in St. Petersburg, Fla., contributed to this report.