Spain's National Court said Inocente Orlando Montano Morales arrived in Madrid on Wednesday morning from the United States and was scheduled to appear in court Thursday.
Montano faces charges of terrorist murder in the killings of the Jesuit priests - five of whom were Spaniards - along with their housekeeper and her daughter.
U.S. court documents said Montano was part of a group of military officers accused of conspiring to kill the priests, who were helping organize peace talks during El Salvador's 1980-1992 civil war. The killings sparked international outrage.
Montano served as El Salvador's vice minister for public security in the 1980s during the Central American country's civil war.
The extradition was welcomed by human rights lawyers who helped persuade Spanish authorities to file charges against Montano in 2011.
"Montano's arrival to Spain brings hope not only to the families and the Jesuit community, but to all victims of El Salvador who have been waiting for justice since the end of the war," said Almudena Bernabeu, a lawyer with The Guernica Group.
Montano arrived in the U.S. in the early 2000s and worked at a candy factory near Boston. He was arrested in 2011 and sentenced to nearly two years for immigration fraud and perjury. A lengthy legal battle over his extradition then unfolded in U.S. courts.
Montano denies involvement in the killings. His lawyer during the extradition proceedings, James Todd, objected to how the Spanish charges and evidence were weighed by U.S. courts.
"He adamantly involves any involvement," said Todd, who won't be directly involved in the Spanish court case. "I certainly hope that the Spanish judicial system responsibly applies due process and carefully examines the allegations."
Todd also expressed concerns about his client's health, describing Montano as a 76-year-old cancer survivor prone to bacterial infections.
Montano, who was most recently held in a South Carolina facility, traveled to Spain on a flight that took off Tuesday night from Atlanta.
Montano's arguments were rejected by U.S. courts, including a federal magistrate in North Carolina who ruled in 2016 that evidence showed Montano took part in the plot. The U.S. Supreme Court removed the final legal hurdle to his extradition earlier this month when it denied a request for a stay of extradition.
Spain has also issued warrants in an effort to try other former officers who are currently living in El Salvador, but the Central American country declined to allow their extradition.
While two officers served short sentences in El Salvador, Montano and other high-level officials were never tried by Salvadoran authorities in the priests' killings. A Jesuit group recently began efforts to have Salvadoran authorities re-examine the killings.
"Today, there is an opening for the opportunity for the truth to be known, to know exactly what happened, so justice may be done. That is what the international community is asking for. That is what the Salvadoran people, the victims, the relatives want," said Miguel Montenegro, head of the Human Rights Commission of El Salvador.
Montenegro said the extradition "fulfills the justice which we have been asking for so much, which has been demanded and has finally been achieved."
Drew reported from Raleigh, North Carolina. AP writer Marcos Aleman in San Salvador, El Salvador contributed.
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