Before the attacks—which were a year in the making—authorities were aware that all but one of the bombers were involved in extremist activities or suspected of other crimes, they said.
A lack of coordination between arms of the government hindered action being taken on the information, officials said.
The attacks showed Islamic State’s influence even in decline, and the ability of a small group of self-organized extremists to act on its message in a country not usually considered a target.
Investigators have drawn these early, still tentative, conclusions, which haven’t previously been reported, about the plot through evidence gathered from the computers and possessions of suspected plotters, caches of munitions, clothing and materials from safe houses associated with them, analysis of the bombs and interrogations of people arrested who either participated in the plot or knew those who did.