If you want a long term answer, grab Brotherhood Mutual Insurance Company’s book “The Church Safety and Security Guidebook.” You can pick it up even if they aren’t the insurance carrier for your church. They have an in depth process to make your building, congregants, and visitors more safe. If your church is willing, in the interim, hire off duty police officers to work during church services if you feel that there is an immediate threat.
Also, there is a book called, “Defending the Flock” that goes through the process of creating a safety team and what you need to do to implement it. Hope this helps and good luck keeping everyone safe.
Edit to add: I’m not affiliated with either of the two organizations whose books I recommended but our church spent the last couple of years getting up and running with these two sources and found them invaluable.