A chemical explosive is set off at a subway station, releasing toxic gas into the air.
This is the scenario for a security drill in the South Korean city of Anyang, as the country's security forces prepare for the upcoming nuclear summit in Seoul.
About 100 police, military personnel and residents took part in the exercise.
They practiced their response to an attack like this in a crowded subway station, evacuating commuters and rescuing casualties.
The Mayor of Anyang said the drill improved safety.
(SOUNDBITE) (Korean) MAYOR ANYANG CITY, CHOI DAE-HO:
"We conducted this security drill as a precaution in order to protect the lives and health of citizens, from possible threats from attackers at various facilities, ahead of the upcoming nuclear security summit."
The summit, will be held at the end of March and leaders from 53 nations will be in attendance.
Simon Hanna, Reuters.