At the starting line in Boston security was very tight, there was a lot of law enforcement and many bomb sniffing dogs. Very unusual. Hadn't happened in previous marathons.
Along the parade route and near the finish line were many men who looked like private security. Not mall cops but Marine/Seal types.
On reddit, the days following the marathon there were many pictures of the unusual security. They knew something was planned.
DHS and the FBI were warned by the Russians
and the perps family ... but ignored them all.
Instead, the terrorists were reportedly let back
into the USA ... without passports.