Pope Francis says nothing can justify the use of the death penalty and there is no "right" way to humanely kill another person.

Francis outlined the Catholic church's opposition to capital punishment in a letter to the International Commission against the Death Penalty, an independent group of former government officials, jurists and others who met with him at the Vatican Friday.

The pope said the principle of legitimate personal defense isn't adequate justification because the death penalty isn't applied to a current act of aggression but "for an act committed in the past."

Declaring "nowadays the death penalty is inadmissible," Francis built on church teaching, including pronouncements during St. John Paul II's papacy, that modern prison systems make executions unnecessary. Francis said there is "no humane way" to execute someone.