Odani sawmill, located in Placas, Pará, linked with logging and processing of illegal timber. An investigation by Greenpeace used covert GPS locator beacons to monitor logging trucks. During the day, empty logging trucks travel deep into the rainforest and at night they travel from illegal logging camps to sawmills in Santarém, These sawmills regularly export timber to Europe, China and the United States. 3°28'21.36"S 54°24'33.05"W. 09/28/2014 © Lunae Parracho / Greenpeace

My colleagues – and friends – in Brazil spent two months placing GPS trackers on illegal loggers in the Amazon. It's dangerous – but it helps us expose their crimes to the world.

Greenpeace activists lived amongst the loggers near Santarém, monitoring their activity at the center of the logging industry in the Amazon. Timber from sawmills there is exported all over the world.

Illegal logging can be hard to come to grips with. Logging happens deep in the forest, far from the eyes of the rest of the world. But all that is changing. Covert GPS tracking technology and satellite images means we can find out what loggers are really up to – and tell the world about it.

Read our report in the illegal logging trade in the Amazon.

A logging truck waits to load timber in an illegally logged area near Santarém, in Pará State, the center of the logging industry in the Amazon. An investigation by Greenpeace used GPS locator beacons to monitor logging trucks. During the day, empty logging trucks travel deep into the rainforest and at night they travel from illegal logging camps to sawmills in Santarém, These sawmills regularly export timber to Europe, China and the United States. 3°25'16.87"S 54°14'10.64"W. 09/01/2014 © Otávio Almeida / Greenpeace

During the day, logging trucks drove deep into the public forest – land owned by the government, where no permission to log has been granted. We photographed them parked in a clearing, surrounded by logs.

As darkness fell, the trucks drove back to sawmills in Santarém.

They carry timber on the roads only at night to evade the police. On one dead-end road – the PA-370 highway – we counted an average of 80 trucks loaded with logs each night. They were all bound for Santarém.

A truck loaded with timber often travel at night when inspections are less common. 08/30/2014 © Otávio Almeida / Greenpeace

Now, there may be a reasonable explanation for why loggers are shuttling back and forth between illegal logging camps and sawmills – and doing it at night. So we checked government records to see where these sawmills claimed the timber came from. Then we used satellite analysis to check those estates and find out how much logging was taking place.

We didn't find very much. In fact, at three of the five sites, we found no evidence of logging at all.

This is a common trick sawmills use to launder illegal timber. They create a fake logging estate to get paperwork that persuades buyers overseas that their logs are legal. It's a well-known scam – we even documented it in a report earlier this year.

Although this story starts in Brazil, it often finishes up in Europe, China or the USA. Companies all over the world are buying timber from these sawmills. Yet European and American laws ban the trade in illegal timber.

As technology and satellite improve and become more available, illegal loggers will be increasing exposed. So will their customers.

Companies trading in timber from the Amazon are taking a massive risk. Some may well be breaking the law.

Until the Brazilian government brings the logging sector in the Amazon under control, buyers need to take responsibility for the wood they're buying, making sure it's been harvested legally and sustainably, or stop buying from high-risk regions like the Amazon.

Click here to view the interactive map at amazoncrisis.org

Richard George is a Forests Campaigner for Greenpeace UK.