BUENOS AIRES, Dec 14 (Reuters) - Cyanide appears to have killed an Argentine coast guard officer just four days before he was to face a verdict on decades-old torture charges, court officials said on Friday.

Police arrested the wife and two children of Hector Febres, who died this week in military custody while being tried for crimes committed at a notorious secret prison known as ESMA during the 1976-83 so-called dirty war.

Court officials, who asked not to be identified, said Febres, 66, had swallowed cyanide. They did not say why his relatives were detained on Friday morning.

Local media speculated that Febres could have committed suicide, given that courts handed down life sentences in three recent cases against rights abusers.

Rodolfo Yanzon, a lawyer on the case, demanded that "those responsible for these crimes be sent to regular jails and stop being held in the arena of their own comrades."

"What has happened allows us to understand very clearly that the Armed Forces are supporting a pact of silence and impunity," he said on local television.

The ESMA held thousands of political prisoners during the seven-year military dictatorship in a crackdown on leftists and dissidents. Many prisoners were drugged and dumped into the River Plate.

Febres was among five Argentine officials sentenced this year to life in prison in Italy, in absentia, for kidnapping and killing three Italian citizens early in the dictatorship.

Argentina has revived human rights trials against members of the security forces during the "dirty war," pressing for amnesty laws and pardons to be overturned since 2003. (Reporting by Lucas Bergman; Writing by Hilary Burke, editing by Doina Chiacu)

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