More mourning and prayers.
Lots of praise for all the first responders.
Less speculation and gossip.
Keep in mind that not all acts of violence are terrorism.
Realize that terrorism can be carried out by individuals, groups, and governments.
Talk to friends and family for support.
Don't spread rumors or more anxiety in any way.
Confront the attention-seeking conspiracists for spreading lies. Their swill is toxic to democracy.
The FBI has not stopped numerous acts of terrorism, they have entrapped numerous hapless people and then busted them for headlines.
Whoever is responsible, do not blame entire groups of people based on their political leanings, religion, or race.