At least four oil tankers — and maybe as many as 16 — got through a U.S. blockade on sanctioned vessels moving oil from Venezuela since the capture of the Latin American country’s president Nicolas Maduro, according to TankerTrackers.com.

The vessels that definitely exited the Latin American nation’s territorial waters were transporting about 4 million barrels of crude and fuel, Samir Madani, co-founder of TankerTrackers.com, which specializes in analysis of satellite imagery, said on January 5. The remainder have 8 million barrels on board, the firm estimates. 

It appeared that all 16 vessels rushed to leave on January 3 after Maduro was captured by U.S. forces, Madani said. Venezuela’s storage facilities are filling up, making it harder for the country to keep pumping oil. A continuation of exports could help to maintain output.

“They happened straight after the Maduro operation and could only have been pulled off if they were done in a wholesale coordinated manner,” Madani said of the tankers’ movements. “That’s the only way to saturate the blockade and make an exit out of the country.”

A lot of ship-tracking focuses on digital signals that vessels transmit, showing where they are going and at what speed and depth in the water. Vessels involved in sanctioned trade sometimes switch off those transponders, making it harder to observe their movements. TankerTrackers.com specializes in examining satellite images.