Transcript for Boston Marathon Bombing: Evil Didn't Win

You've been watching our ongoing coverage on ABC news all day today you've met one of our newest colleagues Byron since we're so happy to welcome -- 20/20 policemen under these incredible circumstances. And you haven't had anything story in a very good movie reunion. Elizabeth thank you know it's it's it's hard to put into perspective. How's it possible that two young men and one of the -- -- you could allegedly do so much harm. Killing innocent people in changing thousands of lives all of it. In an -- The longest week. -- -- -- Attack Monday. -- god lock down Friday. We're getting answers tonight but certain questions always -- one of the hardest one that echoes after every calamity. What crystal Marie Campbell had not taken off this particular day. The restaurant where she works so hard just to cheer and celebrate the spring sunshine. This doesn't make it's. What if a mother and father in China. Have not Scranton saved and -- on their bright daughter thinks he -- Their only child and sending her off to study at a university placed on the other side of the world. One of the little boy with an adorable smile -- rich and his little sister had not gone -- ice -- In Dorchester as a tribute to Martin they stopped the clock. But no one can turn back time. You can ask the questions -- -- 3000 times once for every runner what if so you come to Boston to see your son -- in the Boston Marathon. Absolutely I would epithets. What if Linda -- had not flown all the way from Wisconsin to serious bill tangling running in his first Boston Marathon. She snapped these pictures -- -- turned down the homestretch and then decided at that moment to bolt for the finish line. My son. The finish line. I wanted to hold him and in -- -- I wanted to be there for him that was the moment -- that that was the moment. It was just like that I gotta save you guys I'm gonna go the first class struggle and is on -- was another. Plans. Saw the smoke. Hard for you still. Do you really don't know what to do. And then. Right in front of me there was this -- -- and she just kind of looked like a broken doll. Ironically Linda's son not a -- she's the one who got hurt at that point you know -- -- her pants all ripped out. You know she said she couldn't hear us she lost 50% every hearing in the -- doctors say his temporary. But the memories the Facebook's. Dave will linger a lifetime. And you saw what. Here. -- before the fear for Laura Wellington all those miles and Monday with the last leg of a long journey. The couple years ago if you had ten. You're gonna run a marathon a couple of years I would relax -- Sent gasoline -- dealing -- and around a mild hobbled by bad knee at the halfway point she considered quitting. I mean there were medical tents along the way that I looked out and was like. Brains. Are telling -- to stop my Harris -- just keep going she wonders what -- she had not been slowed by the very good fortune. -- -- -- If you're -- been. OK we'll and we you've got to -- if. My main need was OK I was on -- -- on a four hour and fifteen -- marathon which would have put me at the finish latitude fifty facts. My family when -- finished reading from your parents and you're simply didn't your boyfriend yet to your friends -- -- I don't know exactly where my family is I know there again and so -- in half or five blocks stand on Boylston street. The finish -- waiting for me. Brent Cunningham and finish the Boston Marathon forty minutes before the blast. Blocks away he was still need them the satisfaction. Finishing the 26 mile course for the first time it's a -- -- to run Boston. You -- -- from the did he received a metal to approve. How important that -- -- when you leaned down in this volunteer. Just puts it around your neck it's your little chance to feel like. It is the sense of -- It is a moment Laura Wellington will never know stopped a heartbreaking half miles short of the finish she misses out on a -- -- But words finally reach his -- her family has been spared the. I just how walked over here and just sat down just lost -- -- -- almost on in that instant. Brent Cunningham and his wife passed by. Annexing -- know she takes my blanket his tough talk and that wraps around me and took them out of office snack. Yes did you finish -- that now and look better and said. -- you're finisher. And I just put. And. And we walked away I. This sobbing like I just couldn't get -- anything anything they names like -- Tell him what I -- how much and that's me. Really was -- opportunity to express to her and her moment of loneliness and. That you're not. Before Laura. Could say things are -- get -- -- -- was gone so she -- her mysterious benefactor on FaceBook. I put this thing on FaceBook yesterday just kind of thinking -- a couple of my friends would. Circulated a couple thousand people had shared and then as the 101000 and then this morning I -- dominant of them over a 100000 people she carried out these like I'm available. -- of the post reaches Brent Cunningham by then all the way back home in Sitka Alaska. -- for something like this and online thank you note falls a little short. So we bought Brent a ticket and he agreed to turn around and take another marathon flight back to Boston. We have someone who wants to say hello to you. Love him and it's wrong and what -- Which brings us back to what if of course there is no -- But there is hope. Some would say on Monday evil -- it turns you might feel like -- they might -- won the battle but they're not gonna win the war. Love wins. Game over. This -- spent a week of powerful emotions in Boston. Shock. Anger fear tonight some measure of relief Elizabeth the -- the Boston Globe tomorrow morning makes it best. Nightmare -- here inspired what a fantastic work thank you so much and we'll be right back.

This transcript has been automatically generated and may not be 100% accurate.