ROME – In late July 2013, when Italian Jesuit Father Paolo Dall’Oglio entered a “rebel” territory of Syria, at the time under siege by the Islamic State, he knew something could happen. He want to Raqqa anyway, in hopes of brokering a deal for the release of kidnap victims.

As it turns out, he himself was kidnapped on the 29th. No one has heard of him since.

Four years later, his sister and Italian civil authorities are remembering him as a “bridge builder,” a man who served for 30 years in Deir Mar Musa, a 6th-century monastery 50 miles north of Damascus.

He’s credited with single-handedly transforming this desert outpost of Syriac Catholicism into an internationally celebrated inter-faith cultural center.

Some believe unconfirmed claims by opposition sources from Raqqa reporting Dall’Oglio was excuted by ISIS soon after his disappearance, with his body thrown into a mass-grave called “Al-Houta.” Others, however, maintain hope that Dall’Oglio may still be alive, held incommunicado, and are pressing for clarification.

Italian President Sergio Mattarella released a statement on Saturday expressing “closeness and solidarity” to the family, “so tried by a long and painful wait.” He defined the Jesuit priest as a “a symbol of dialogue between religions.”

Italy’s Prime Minister Paolo Gentiloni, on the other hand, refrained from speaking about his death, instead sending out a tweet on Saturday that read: “Four years after his disappearance, a thought for Father Paolo #DallOglio, who disappeared in Raqqa, and for his family. We continue to work and wait.”

Described by many not just as a priest but also an activist focused on Christian and Muslim dialogue, Dall’Oglio supported the uprising against Syrian President Bashar al-Assad, which is why he was expelled from the country in 2012. His commitment to inter-religious relations earned him the Nobel Missionary Prize, which was awarded by the Cuore Amico Fraternità Missionary Association in his absence in 2014.

At the time of his disappearance, he’d reportedly walked into the ISIS headquarters in Raqqa to negotiate the release of hostages and to broker a truce with Kurdish militias.

Dall’Oglio’s sister, Immacolata Dall’Oglio, spoke with Vatican Radio on the eve of the anniversary of his disappearance. She said that silence regarding what happened to him is “total,” despite the many “voices” asking.

The family continues to wait, she said, hopeful despite the risk of expecting too much after four years. For them, according to the priest’s sister, “this silence is a reference to the need to know the truth, the only way to awareness, justice and to be able to look beyond.”

On Saturday, the family organized a Mass in Rome in honor of the missing priest, in an attempt also to shine a light on what is happening in Syria, a prayer for peace to return both to this country and neighboring Iraq, that’s also been devastated by ISIS.

These countries can achieve peace, Dall’Oglio said, if they learn from the past and their mistakes, and if they follow the path left by the priest, who went through life “experiencing a full trust in God.”

She defines her brother as a man of faith with a “special gift,” capable of relating to another person’s “most profound aspects.” She also believes that he must have built “significant relations,” with many, particularly Muslims in Syria.

“Otherwise, we wouldn’t be able to explain the sharing, the closeness and the solidarity that we’ve experienced in these four years from the most unexpected people who were touched by him,” she said.

Speaking about Pope Francis’s appeal two years ago for the release of the priest and everyone else who’s been kidnapped in Syria, she said that “every initiative is a sign that Paolo is not alone, and that despite the war, the seed of dialogue was planted and it’s sprouting, confirmed as the only possible path.”

The pope’s appeal took place on July 26, 2015, at the end of his Angelus address, where he prayed for the release of Dall’Oglio and two Orthodox archbishops who were taken captive in April 2013. He asked the faithful “to remember them in our prayers.”

Speaking to tens of thousands of people in St. Peter’s Square for his weekly prayer and message, Francis called for a “renewed commitment” by local and international authorities “so these brothers of ours can regain their freedom soon.”

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Kidnappings and ransoms have become common occurrences among extremist groups such as ISIS, who frequently take hostages to instill fear and to use them in order to raise money to fund their so-called caliphate. Many are freed once ransom is paid, but many more never make ransom, or were taken by groups who never reached out for one, as in the cases of Dall’Oglio, Aleppo’s Syriac Orthodox Archbishop Mar Gregorios Yohanna Ibrahim and Greek Orthodox Archbishop Boulos Yazig.

In February 2013, the website “Ora Pro Siria,” operated by Italian missionaries in the country, launched an emergency fundraising appeal it called “Ransom a Christian.” According to the website, the going price for a kidnapped priest in Syria at that time was in the neighborhood of $200,000.

The two bishops were kidnapped as they were on the way to negotiate the release of two other kidnapped priests, Father Michel Kayal and Greek Orthodox Father Maher Mahfouz, both of whom were taken captive Feb. 9, 2013.

None of the clerics have been heard from since their disappearances.

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Back in January, during a speech to Catholic and Oriental Orthodox leaders, Pope Francis drew specific attention to those who have suffered due to violence, saying “my heart goes out” in a special way to the bishops, priests and laity, particularly children and elderly, who have been “cruelly abducted, taken hostage or enslaved.”