1st clues suggest attack may be case of domestic terror

Event affords easy access to extremists

We have disabled comments on this article.

You have reached the limit of 10 free articles a month

Stay informed with unlimited access to Boston’s trusted news source.

  • High-quality journalism from the region’s largest newsroom
  • Convenient access across all of your devices
  • Today’s Headlines daily newsletter
  • Subscriber-only access to exclusive offers, events, contests, eBooks, and more
  • Less than $1 a week