Police were called to the media centre on Thursday after a man, who appeared distressed, walked into the media room before any players or team staff arrived for their media duties.
The man, who attempted to leave the premises after hearing nearby unrelated sirens, was ushered away and disarmed before later peacefully departing. The FA will now review the incident.
At no point were players or coaches under threat, with only FA staff and journalists in the media centre at the time.
The media centre, set in a community building around half a mile from the training pitches at Swope Soccer Village, is protected by security guards.
England are currently preparing for a World Cup quarter-final meeting with Norway, which will take place in Miami on Saturday evening. They will fly to Florida on Friday.
Jarrell Quansah will be unavailable for the quarter-final after he was sent off against Mexico and subsequently slapped with a two-match ban, which will rule him out of the World CUp until the final, provided England reach it.
It remains to be seen, too, whether Reece James, Marc Guehi or Declan Rice will be available to feature.
Full-back and Chelsea captain James has been absent since England’s second group stage match with a hamstring injury, while Guehi missed training this week with a similar injury. Rice is dealing with an illness.
Any prospective victory against Norway will set up a semi-final clash with either Argentina or Switzerland.