Federal investigators say they believe the man who carried out a mass shooting at Brown University and later killed a Massachusetts Institute of Technology professor did not act randomly.

Instead, former Brown student Claudio Neves Valente, 48, appeared to target places and people for what they represented in his own life – institutions and individuals he associated with personal failure, missed opportunity and perceived injustice.

In a detailed behavioural assessment released on Wednesday, the FBI said Neves Valente, a Portuguese national, spent years planning the attack in isolation before killing two students and wounding nine others inside an engineering building on December 13.

Two days later, he killed MIT professor Nuno F.G. Loureiro at his home in Brookline, Massachusetts. Neves Valente was later found dead of an apparent self-inflicted gunshot wound inside a storage facility in Salem, New Hampshire, ending a multistate search.

The FBI described a man who spent years in isolation, rarely staying in one place and lacking traditional support systems such as family, peers and authority figures who might have recognised warning signs and alerted law enforcement.

Over time, investigators said, he built a narrative of grievance and inadequacy, with “little to no opportunity for bystanders to observe and contextualise the significance of his behaviours”.