Twenty bronze-capped cobblestones, commemorating members of two Italian Jewish families who were deported during the Holocaust, were stolen in Rome in the early hours of Monday morning.

The memorials were set into the street in front of the former homes of the two families, in Rome’s Monti neighborhood, not far from the Colosseum.

Police are investigating the Dec. 10 theft, which has left a gaping hole in the street, as a hate crime.

The cobblestones marked the name, date of deportation or arrest, and if known, the place and date of death, of 20 Italian Jews. Among those memorialized in the stolen stones were 18 members of the Di Consiglio family and two members of the Di Castro family.

Fifteen were deported to Auschwitz during World War II, dying either there or in an unknown place.

The other five were killed by the occupying Nazis in the Ardeatine Massacre, carried out in caves outside Rome. They were among 335 Italian men and boys assassinated in March 1944 in retaliation for partisans having killed 33 German policemen.

At a news conference Monday, Italy’s deputy prime minister and interior minister, Matteo Salvini, called it an act of “repugnant anti-Semitism” and said he would work to stop similar acts.

The group of plaques were incorporated into the classic Italian “sampietrini” stones which make up the side and pedestrian streets of cities. There are around 200 of these memorials in Rome, sometimes called “stumbling stones” because of their ability to provoke thought in passers by.

German artist Gunter Demning started the initiative of placing these memorials, called Stolpersteine, outside the homes or workplaces of Holocaust victims in Europe in 1992.

The mayor of Rome, Virginia Raggi, said in a tweet Dec. 10 that the theft of the stones was “unacceptable” and “a gesture that I condemn with force and deep indignation. Memory demands respect.”