Fortunately for him and his family, they were already on their way back to their hotel when bombs went off.
"We left the area 20 minutes before it happened. We heard the sound in the cab. We thought it was a car or train accident," he said.
When they got back to their hotel, which was between Miles 23 and 24, they stood on the street cheering for the runners still on the course. Soon, however, everyone was told to clear the streets and go inside.
It wasn't until he and his family got up to their room and turned on the television they found out what happened.
Hennessey called those who set off the bombs the "lowest of the low."
"They took a great event, did it at the average time, 4 hours, they did it when they knew people be down there [at the finish]. I think they're cowards and they're horrible," he said.
'This isn't real life'
Erin Schisler of Bel Air doesn't feel any of the pride and achievement she should have felt after finishing one of the best-known and toughest races in the world.
It was her first Boston Marathon, and Schisler was elated that she had qualified for it. She finished in 3:57:15.
"It was going to be one of the most exciting weekends of my life, except from getting married," she said from the Boston airport Tuesday afternoon.
"You really can't feel pride in your own achievement when people had to lose their life during that," Schisler said. "There are truly no words to describe how it feels. The feeling of sense of pride and achievement are not among the feelings I have right now."
The 29-year-old teacher mentor at Hall's Cross Roads Elementary in Aberdeen and Homestead-Wakefield Elementary in Bel Air had a rough run, and was being treated by medics at the end of the race. The medics, she said, wanted to her to go the medical tent, but she didn't, because she hadn't gotten her medal yet.
Schisler recovered enough to get herself back to her hotel, which is where she was heading to meet her family. But when she tried to call and text, she couldn't get through.
Her husband, Tom, her dad, Pat Cahill, and her brother, Ryan Cahill, of Reston, Va., finally found her and got her back to their hotel so she could rest, without telling her there had just been an explosion at the finish line.
She got in the elevator with some other runners and one asked if they had heard what happened. When Schisler heard, all she could think was "My mom's out there."
"I got back to my room, dropped my stuff on the floor and tried calling," she said. "I thought, I was just there, this isn't real life."
It took 45 minutes to find out her mom, Pam Cahill, was OK.
"It was the most scariest event of my life," she said.
Her husband, dad and brother heard the first explosions from their spot on the course, then looked up the street and saw the second one. Her mom had just crossed the area not too long before, but was around the corner when the blasts went off.
"I just feel absolutely terrible for the families and victims. You hear stories of runners whose entire family was impacted. These people came out to cheer us on and literally the entire race they were best spectators," she said. "For someone to come in and take away that trust and unity is absolutely shocking. I feel sad, like they robbed Boston of one of its finest events."
The worst feeling, she said, was not feeling safe.
"It's a feeling you don't really experience. We didn't know if we were safe where we were, if we should be in the city or out of city. Then public transportation shut down. We didn't know what to do, do we venture out? We didn't know where to go, what to do. We didn't know how to feel safe," Schisler said. "That feeling was very temporary, but for the time it lasted, it felt like a lifetime."
Aegis running correspondent Bill Blewett also contributed to this article.