What if you had just one opportunity to say goodbye to a loved one immediately before you died? Would you know what to say? Would you be able to bring yourself to say it?
On September 11, many of the more than 2,800 people killed in the attacks on the World Trade Center faced those very questions.
The attacks took place as thousands of workers were just starting their workday.
Moises Rivas, 29, a chef at Windows on the World, the renowned restaurant at the top of the center's North Tower, was able to get a call out immediately following the attack.
His stepdaughter Linda Barragan answered, according to Elizabeth Rivas, Moises' widow.
"So I called Linda and said, 'Linda, did Moises call?' And she said, 'Yes, Mami, he said not to worry, he is OK.'
Moises Rivas never called again, leaving his widow with a secondhand account of his message of love.
"A lot of people, maybe they were screaming and crying and getting suffocated with the smoke and everything and he tried to call me," Elizabeth Rivas said. "He called me."
International trade consultant Melissa Harrington Hughes, 31, also stuck in one of the towers, called her father Bob Harrington at home in Massachusetts at 8:55 am, nine minutes after the first plane crashed into the North Tower.
"She was a little hysterical and I couldn't understand what she was saying so I said, 'Slow down a minute and tell me what the problem is so I can help you out,'" Bob Harrington said.
"I said, 'You get to the stairwell and get out of that building as fast as you can.' I told her that I loved her," he said. "She said, 'I love you too, Dad' and she said, 'You have to do me a favor. You have to call Sean and tell him where I am and tell him that I love him.'"
Twelve minutes later, at 9:07 am, Melissa was able to make a second call to her newlywed husband Sean, who was asleep in San Francisco, and leave a message.
"Sean, it's me," she said in her message. "I just wanted to let you know I love you and I'm stuck in this building in New York.
"There's a lot of smoke and I just wanted to let you know that I love you always," she said.
Her father was left to live with a gut-wrenching image, yet he was one of the few that got to say good-bye.
"When she called me, she was panic-stricken but she thought she could get out of the building," Harrington said. "But when she talked to Sean I could see in her voice that see knew she was going to die.
"In one instance, it's really good that I talked to her," Harrington said, weeping. "Because I can always remember us exchanging 'I love you' - 'I love you, Dad,' It's just painful sometimes because you just don't forget a girl like that."
'We're trapped on the 106th floor'
Other messages were received from victims via the Internet.
Bill Kelly, 30, used his Blackberry pager to send an e-mail at 9:23 am to his sales manager at Bloomberg L.P., who had e-mailed him to ask if he was safe.
Kelly's sister Colleen said she became fixated with the messages from her brother.
"I was really obsessed with [the] messages," she said. "Really obsessed and [I] really wanted to know everything that Bill might have communicated.
"I think, for me, it was helping me accept his death and accept that he wouldn't be able to communicate anymore with us."
Another sister, Mimi, said what she values most about Kelly were the "messages he gave us all throughout his life."
"We're never going to know exactly what happened," she said. "And I've resolved in that I know he probably died the way he lived his life.
"He died with honor, he died with courage, he died a gentleman and he died with a lot of love and faith because that's how he lived."
'She said, I love you'
Outside Washington on September 11, American Airlines Flight 77 was hurtling toward the Pentagon while passengers were herded into the rear of the passenger compartment.
First, Barbara Olson told her husband that her plane had been hijacked.
"We then both reassured one another, this plane was still up in the air, this plane was still flying," Ted Olson recalled during an interview broadcast on CNN's "Larry King Live" on December 25, 2001.
"And this was going to come out OK. I told her it's going to come out OK. She told me it was going to come out OK. She said, 'I love you. ...'
"We segued back and forth between expressions of feeling for one another, and this effort to exchange information. And then the phone went dead."
Flight 77 crashed into the Pentagon, killing 184 passengers, crew members and people inside the Pentagon.
Another hijacked airliner, United Flight 93, also carried passengers who phoned loved ones.
Mark Bingham, 31 -- one of a group of passengers who tried to overpower the hijackers and regain control of the plane -- called his mother to say goodbye.
"He said, 'I want you to know I love you very much, and I'm calling you from the plane. We've been taken over. There are three men who say they've got a bomb,'" recalled his mother, Alice Hoglan.
Fifteen minutes later, Flight 93 crashed in a field in rural Somerset County, Pennsylvania, killing 40 crew members and passengers.