CHRISTCHURCH, New Zealand—This tiny island nation was left reeling after a gunman wearing military fatigues and wielding five weapons stormed into two mosques and killed 50 people on Friday, in the worst massacre in New Zealand’s history.

Police charged a 28-year-old Australian suspect, Brenton Tarrant, with murder. Authorities arrested two other suspects, as they try to determine whether they were involved in the massacre.

A...