Me [28 M] with my colleagues [20sM/F -50sM/F) and... our coworker [20sM?] who we're afraid is going to harm us

I don't know what sub to post in. I am very scared and r/relationships has helped me before... and I guess this does involve an interpersonal problem.

Well, I never thought I would be in this situation. I have a normal technology-focused workplace with a lot of normal, functional people and pretty light on the drama (usually).

One of my colleagues, Milton, is a little high maintenance. He is a generally nice guy but he sometimes gets a little overanimated about things. He has made some of my colleagues a little uncomfortable in meetings in the past.

A month ago, we had a staff meeting in which Milton got very weird. He was silently fuming about something and looked full of rage. The meeting didn't specifically involve him but he seemed extremely frustrated about something. Again, some people were a little uneasy.

That same week, later on in the week, Milton got worse. He started behaving very weirdly, not responding to people when they spoke to him, walking around the office strangely/creepily, and failing to show up for meetings. The staff got really worked up at this point and approached the manager to express concerns.

The manager talked to Milton and (not sure what happened, wasn't in this meeting, some of this is third or fourth hand) Milton was asked to take a couple of weeks off from work to just relax and take care of himself.

This week at work we received a birthday card in the mail, addressed to "Business Name, Floor #, City, State." The birthday card was blank and smeared with blood.

EVERYONE is terrified. We can't prove that he sent the card, but Milton has now been asked not to return to work and the police have been involved, but that is as far as corporate has taken it so far. We are afraid he is going to turn up on our floor (or at our company picnic, in a couple weeks) and hurt us and we don't know what to do.

How do I help keep people calm? What do I do? I am afraid to go to work too but I'm even more worried about some of my colleagues, who are still expected to come to work every day and many of whom are having a REALLY hard time handling this situation.

tl;dr: Coworker appears to have snapped. Workplace got a restraining order or something, but the staff is a mess and everyone is terrified. What do?