The robbery is the biggest in Chilean history.
The gang held up the guards unloading the money from a van belonging to US security firm Brinks at a cargo area at Santiago's international airport.
The cash had been destined for delivery to banks and mining operations in Copiapo, in northern Chile.
Security footage showed the gang arriving disguised as workers in a white van at 06:21 local time (10:21GMT).
The robbery itself was not captured on camera, but the footage shows the same van leaving the airport two minutes later.
Chilean Interior Minister Mahmud Aleuy said security at the airport was "an embarrassment".
He said the security measures should be in proportion to the value of the goods transported.
Security at the cargo area is handled by the civil aviation authorities, not by the police, he said.
The robbers fled in two vans in opposite directions, scattering nails on the road in their wake, local media reported.
One of the vans was later found empty and abandoned in a fallow field.
The size of the loot surpasses that of a similar robbery in 2006, when robbers stole $1.6m from a Brink's delivery at the same airport terminal.
The men behind that robbery were eventually captured and are serving time in prison.
Interior Minister Rodrigo Penailillo said the gang which carried out Tuesday's robbery was "highly organised".
He said the Ministry of Defence would help the police track those responsible.
Eight masked gunmen held up an armoured money transport in the Chilean capital Santiago on Tuesday, making off with more than $10m (£6.2).