CONAKRY (Reuters) - Senegal authorities have arrested a Guinean soldier linked to a 2009 massacre in Conakry where at least 150 people were killed and dozens of women raped, a Senegalese security source said on Tuesday.

The source said Lieutenant Aboubacar “Toumba” Diakite, who witnesses say played a key role in the massacre at a Conakry stadium, was arrested in Dakar on Monday.

The Sept. 28, 2009, incident in Guinea’s capital is seen as one of the worst acts of repression in West Africa’s recent history and Human Rights Watch called the arrest a breakthrough in the bid to bring justice.

In that incident, security forces opened fire on pro-democracy protesters who had gathered at the stadium to put pressure on then junta leader Moussa Dadis Camara not to stand at an election the following year.

Diakite was head of the presidential guard and a close aide to Camara, who took power in a coup in 2008.

“The arrest ... represents a major step forward in Guinea’s investigation of the 2009 stadium massacre,” said Corinne Dufka, an associate director at Human Rights Watch. “Victims are eager to see the case move to trial.”

An extradition process has started to bring Diakite to Guinea, said Baila Diallo, a public prosecutor at Guinea’s Court of Appeals.

Diakite is also accused of having organized mass arrests of army officers seen as hostile to the junta. Many were tortured, according to their families.

Diakite shot Camara in the head in an assassination attempt in Dec. 2009 and told French radio he did it because Camara had cast the blame on him for the stadium massacre.

Camara survived but resigned the presidency and fled to Burkina Faso, where he was charged in 2015 for crimes relating to the massacre. He has not yet stood trial.

President Alpha Conde won the 2010 election in the first democratic transition in Guinea, the world’s top exporter of aluminum ore bauxite. He was re-elected in 2015.