In one of its most audacious attacks yet, the Movement for the Emancipation of the Niger Delta (MEND) attacked an offshore oil rig for the first time, forcing Shell to shut down the entire oil field and causing a 10% reduction in Nigeria’s oil output.

Before the attack, Nigeria’s output was already 20% below normal following a sustained campaign by MEND to target the nation’s oil extracting infrastructure. These attacks have been identified as a major reason for high global oil prices.

The Bonga offshore oilfield normally produces 200,000 barrels a day, and is Shell’s flagship offshore project. To date, it has been considered too difficult for militants to attack. But early on June 19, a boat full of gunmen raided the installation, damaging a storage and offloading vessel and taking a US worker hostage. The worker was released several hours later.

In addition to the sheer economic impact, the attack is considered an important symbol that no region of NIgeria’s oil production is beyond MEND’s reach.