SANTIAGO (Reuters) - The former head of Chile's army was detained on Thursday as a formal investigation opened into his alleged involvement in the killing of 15 people during the first days of the nation's bloody 1973-1990 dictatorship.

Retired General Juan Emilio Cheyre, who led Chile's army from 2002 to 2006, after the return to democracy, was a junior military officer at the time of the 1973 coup.

During his time as chief of Chile's army, Cheyre famously said a coup and humans rights violations should "never again" happen in Chile.

Cheyre is accused of complicity in the killing of 15 people in the northern city of La Serena on Oct. 16, 1973, during the "Caravan of Death." Eight other ex-officers from the same regiment are also being investigated.

"Judge Mario Carroza opens investigation into retired general Juan Emilio Cheyre and eight other people for Caravan of Death case," Chile's judiciary said in a tweet.

The "Caravan of Death" was a military committee that traveled around Chile by helicopter after the coup, ordering the deaths of suspected leftist opponents.

(Reporting by Anthony Esposito; Editing by Peter Cooney and Sandra Maler)