Former Guatemalan police chief Erwin Sperisen appeared in a Geneva court Monday as he faced his fourth Swiss trial over the extrajudicial killings of seven prison inmates nearly two decades ago.

Sperisen -- who holds dual Swiss and Guatemalan citizenship -- has over the past decade been convicted three times in Switzerland over the 2006 killings in the Pavon prison near Guatemala City, only to see those rulings overturned.

At the opening of his fourth trial, his lawyers demanded he be acquitted and that the case be definitively thrown out, according to the Swiss news agency Keystone-ATS.

Sperisen, who has always maintained his innocence, has already spent 11 years deprived of liberty, split between preventative detention and house arrests, the news agency said.

Sperisen, nicknamed "The Viking" due to his hefty build and ginger beard, resigned as Guatemala's police chief in 2007 and moved to Geneva, where his father Eduardo Sperisen-Yurt serves as Guatemala's ambassador to the World Trade Organization.

He was arrested in 2012 over the alleged summary execution and subsequent cover-up of the murder of seven inmates in Pavon jail six years earlier, including one he allegedly shot dead himself.

He was initially sentenced by a Geneva court in 2014 to life in prison, but in 2017, Switzerland's Federal Court ordered a retrial over what it deemed was the use of faulty evidence.

A Geneva appeals court sentenced him again in 2018, this time to 15 years behind bars.

The federal court confirmed that verdict in 2019 -- but reversed its decision last year following a ruling by the European Court of Human Rights condemning Switzerland over what it determined was a lack of impartiality on the part of one of the Geneva judges in the case.

The federal court ordered Geneva's appeals body to re-examine the case once again.