Mexico held its annual earthquake drill just two hours before it was hit by its second tremor in a month, killing more than 200 people and toppling dozens of buildings in the capital, Mexico City.
The Latin American nation staged its emergency disaster simulation at 11am on 19 September as it has every year since 2004.
The drill falls on the anniversary of the country's deadliest tremor, an 8.1-magnitude earthquake that shook Mexico's capital in 1985, burying nearly 10,000 people amid its rubble.
On Tuesday the alert to begin the drill went out over radio, television, phones and public loud speakers. Around seven million people across the country's major cities left their homes, offices and shops and headed to designated safe areas promoted days ahead of time.
Many Mexicans treat these drills as a way of life, or even a minor annoyance.
This year five states opted out of the drill after a 7 September an 8.1 magnitude earthquake struck off the country's southern coast, triggering tsunamis and sending shockwaves as far as Mexico City and Guatemala City.
These districts said they would suspend drills as a mark of respect to those still recovering from the disaster, and to avoid alarming residents.
But a little over two hours after the drill on Tuesday, at just past one in the afternoon, another 7.1 magnitude quake struck, causing major damage in Mexico City and neighbouring states.
The epicentre of the quake was near Atencingo in Puebla state, about 120km (75 miles) from Mexico City, at a depth of 51km, the US Geological Survey said.
For a second some thought this was another drill, but soon realised a second natural disaster had hit the country in September.
Dorothy Munoz was watching a television special in her Mexico City apartment marking the 1985 catastrophe when the ground began to shake, she told CNN.
She said a fish tank fell to the floor along with decorations and furniture. She grabbed her dogs and headed for the streets where people had gathered. Officials told them to wait to return to their apartments until buildings were checked for structural damage.
However, lessons learnt from the 1985 disaster led to changes in building codes and enhanced emergency preparation measures, including the annual drills. Over the years these have led to fewer deaths, injuries, and building collapses in a country that is prone to earthquakes.