It’s a split decision that could save your life in a terror attack — do you run away, or play dead?

The answer, according to new guidance from the UK’s National Counter Terrorism Security Office (Nactso), is to run, or hide behind “substantial brickwork” or “heavy reinforced walls.”

Which in the cold light of day, could sound pretty intuitive. Except, for some of the people caught up in the Paris attacks on Friday, it wasn’t.

French raid in Saint-Denis
French raid in Saint-Denis

The new guidance has been released after some of those at the Bataclan Music Hall reportedly lay down and remained still, while terrorists indiscriminately fired at the crowd.

Of the 129 people who lost their lives in the Paris attacks, 89 were music fans who had gone to see band “Eagles of Death Metal” perform.

If you do find yourself in the middle of attack, the report advises following three principles:

1.) Run:

Identify the safest route and escape if you can. Insist others come with you, and leave belongings behind.

2.) Hide:

If you can’t run, then hide — try and find a secure hiding place, ideally behind heavily reinforced walls. And remember that bullets can travel through glass, wood, brick, and metal. Also make sure that mobile phones are turned to silent.

3.) Tell:

Once you’re able to, call the police and give them as much information as possible — where are the suspects and what do they look like? How many people remain inside the building?

As reported by CNN