FRANKFURT, Germany (AP) — Cologne police acting on indications of a possible attack searched Germany’s landmark cathedral with sniffer dogs Saturday and said worshippers attending Christmas Eve Mass would undergo security screening before being allowed in.

In Austria, police in Vienna also said they were taking heightened security measures around churches and Christmas markets, deploying both uniformed and plainclothes officers.

Neither police agency specified the threat, but the German news agency dpa said authorities were responding to signs of a possible attack by Islamic extremists, without citing a specific source.

Michael Esser, head of the criminal investigation department of the Cologne police, said in a news release that the threat indications pointed to New Year’s Eve rather than Christmas, but added that “we are putting everything possible in motion for the security of cathedral visitors on Christmas Eve.”

Cologne’s towering cathedral, whose twin spires are 157 meters (515 feet) tall, is a major tourist destination visited by some 6 million people a year.

Police and cathedral officials urged those attending Mass on Sunday evening to arrive early and not bring bags or purses.

The European Union’s home affairs commissioner, Ylva Johansson, warned on Dec. 5 that Europe faces a “huge risk of terrorist attacks” over the Christmas holidays due to fallout from the war between Israel and the Palestinian militant group Hamas.