A Russian court has placed four private oil firm executives under arrest for what authorities say is their role in the tainted oil scandal that forced Moscow to shut a major pipeline to central Europe and Germany.

As of Tuesday, the main route of the Druzhba pipeline had been closed for almost two weeks and it was unclear when normal operations would resume, sending shocks through global oil markets and damaging Russia’s image as a reliable supplier of energy. Pipeline monopoly Transneft said the oil had been contaminated deliberately.

Four chief and assistant chief executives of the little-known Nefteperevalka, Petroneft Aktiv and Magistral oil firms have been arrested, Russia’s Investigative Committee said in a statement. Two more suspects were placed on the wanted list.

Investigators say the suspects had illegally pumped low-quality oil near the Volga River city of Samara “to conceal thefts” sometime between March and April this year.

Svetlana Balabay, Rustam Khusnutdinov, Vladimir Zhogolev and Sergei Balandin will remain in pre-trial detention until June 29, Interfax reported.

They face three charges each of damaging crude oil pipelines, creating an organized crime group and theft.

Balabay heads Nefteperevalka and Khusnutdinov is identified as its deputy; Zhogolev is the general director of Petroneft Aktiv and Balandin is the deputy head of Magistral.

Nefteperevalka, according to reports, owns the section of the Druzhba pipeline where it is believed the contamination had originated on April 19.