(ABC News) - The first time Lisa O'Brien knew something bizarre was happening was when she found her 4-year-old daughter Jacie having a conversation in an empty room.
"I'd say, 'Jacie, who are you talking to?' 'My dad! Talking to daddy,'" O'Brien recalled. "She would tell me she could see him."
Jacie's father had recently died in the 9/11 attack on the World Trade Center. O'Brien said she tried to gently probe Jacie, who revealed that her daddy had been telling her "knock, knock" jokes. Jacie also could recite the names of her father's co-workers, who had all died in the attack -- many of whom her mother said she had never met.
"I showed her a picture and said, 'Do you know who these guys are?', and she said, 'Mommy, those are the boys,'" said O'Brien, who was in awe of her daughter's connection with her deceased dad. "I was jealous…because she got to see them and I didn't."
Watch the full story on "20/20" Friday at 10 p.m. ET
The O'Briens are just two of many people coming forward in what may be the strangest aftermath of 9/11. For most, the attack on the World Trade Center remains a national and emotional scar, but for some relatives left behind it has also been a step into a spiritual world they say they never could have imagined.
Bonnie McEneaney's strange journey began with premonitions. Her 46-year old husband, Eamon, a legendary athlete turned trader who worked at the World Trade Center, seemed to have a foreboding feeling in the weeks before September 11th. On September 2, 2001 she said they were at a barbecue where Eamon spoke to his brother about the possibility of the World Trade Center being hit again. He was debating whether he should take people to the roof this time or whether he should try to bring them down the stairs, McEneaney recalled. And in the days that followed, she said it was clear that Eamon suspected he didn't have long to live.
"One night, I was cooking, he walked in and he said, 'You had better become more of a disciplinarian with the children because when I'm gone it's going to be hard,'" McEneaney said.
The morning of 9/11, as Eamon got ready to go to work, his wife said there was another sign. In the shower, Eamon had a vertigo attack -- a problem he had developed years before but hadn't affected him in over nine months.
Speaking with others in the 9/11 community, McEneaney discovered that "goose-bump" experiences like hers were common.
"I think there's probably been close to 200 people that I've interviewed or had discussions with," said McEneaney, who turned their stories into a book called "Messages: Signs, Visits, and Premonitions from Loved Ones Lost on 9/11."
Once a doubter, the former corporate vice president now says she is a firm believer in premonitions. She points to stories like Monica and Michael Iken's. The couple got married on September 11, 2000, and Monica said Michael felt that something was off that morning. During the outdoor service, a jet buzzed overhead, so loud and so close, that they had to stop the service precisely at "I do."
"He just was horrified. He turned to me and was like, 'We're jinxed,'" Monica Iken recalled.
Nearly a year later, on September 9, 2001, the Ikens checked into a hotel at Boston airport.
"He's like, 'Monica, I have to get out of here, right now, we have to check out right now,'" she said.
Monica couldn't figure out why her new husband was so distraught, but after he was lost in the Twin Towers, she learned that the hijackers had been casing out the airport at that same moment. Afterwards, when she looked at one of her wedding photos, she noticed another eerie discovery: a black streak covered Michael's whole body in one of the photos.
Some had even stronger premonitions. David McCourt's wife Ruth was supposed to head to the west coast for a brief trip, but he said Ruth seemed to know she wasn't coming back. "She was writing notes in her final week to people that she cared for, making amends with everybody," McCourt said. Their 4-year-old daughter Juliana was supposed to stay home with David that day, but at the last moment, his wife changed her mind.
"She turned to me and said, 'David, Juliana belongs with me. I'm taking Juliana with me,'" he said.
Both Juliana and Ruth were on one of the doomed flights that crashed into the towers.
Could these premonitions be real?
"I think people feel that they experience premonitions," said Dr. Katherine Shear, a psychiatrist and expert on grief at Columbia University. "What we don't know is how often people experience premonitions that actually come true, compared to ones that don't."
But many of the people coming forward say there is no rational explanation for what they went through the terrorist attacks. The morning of 9/11, McEneaney had her first strange experience in her front yard. Still hoping against hope that her husband had survived, McEneaney went outside and yelled, "Eamon where are you?" All of a sudden she said she heard leaves rustling and the branches swaying and "a river of wind" slowly swirled and twirled toward her. It whimsically lifted her skirt and then stopped. She knew at that moment, she said, that her question had been answered. Eamon was gone.
McEneaney said there are so many stories of people who thought they were alone in these eerie experiences until they opened up to each other. Lisa O'Brien and her friend Joanne Kelly bonded over their strange experiences.
O'Brien recalled a ritual she shared with her deceased husband: "He was whistling 'A Penny For Your Thoughts, A Nickel For A Kiss,' and I said, 'Wow, I wonder what a quarter would be.'"
Quarters became the couple's private joke. After he died, O'Brien said she started finding quarters everywhere, and in the strangest of places.
"I would get up out of bed in the middle of the night to check on the kids and I'd come back to bed and there'd be a quarter in my bed," said O'Brien, who is convinced that the coins were a message from beyond the grave.
Kelly was on the phone with her husband when the first plane hit the North Tower. Watching the television, she saw the second plane hit. At that moment, she said, "I couldn't feel him." Much later, she toured the morgue, where medical examiners had been thus far unable to locate her husband's remains. As she stood there, she said, she felt her heart racing. There were eight morgue trucks lined up and Joanne pointed to one and said, "He's in that one." As if on cue, she said, a staff member came into the room and said, "You're never going to believe this. This never, ever happens, never happens, one in a million. We found his remains."
Even more astonishing are reports of actual sightings. Monica Iken recalled waking up, to see her husband Michael standing at the foot of the bed.
"He was all glowing, and I just sat up, I said, 'Thank you for coming.' As quick as I said that, he left. But he was smiling. He was telling me he was there with me. It was not a dream. He was literally standing there," Monica Iken said.
"These kinds of things tend to happen with people when people lose someone very, very close to them," said Shear, who added that while there are rational explanations, she doesn't totally discount the possibility that the incidents might be something more. "Obviously, there's so many things that we just do not understand."
In the end, McEneaney and the others said they don't mind whether you believe them or not; the messages they have received brought them comfort they deeply needed.
"It's not about death," said McEneaney. "It's about love and hope and loving connections that continue, even after death."