PARIS (AP) — Special anti-terrorism measures being put in place to safeguard the unprecedented opening ceremony for the Paris Olympics on the River Seine will also apply to all buildings along the route, meaning people who work and live there and their guests will be subjected to background security checks, Paris police chief Laurent Nunez said Thursday.

Those affected will be cross-checked against security services’ databases, to see whether they have previously been flagged as suspected Islamist extremists or for other radicalism, Nunez said.

The wildly ambitious July 26 ceremony is proving to be a gargantuan security challenge. Athletes will be paraded through the heart of the French capital on 94 boats along a 6-kilometer (nearly 4-mile) stretch of the Seine, from east to west. They’ll be accompanied by 87 other boats for security, media and other people.

All of the parade route will be inside a high-security zone that Nunez described as an “anti-terrorism perimeter.” He said it will include the first row of buildings along the route.

“We wanted to include in the perimeter all of the buildings that have a view on the parade,” Nunez said.

The zone will be put in place from July 18 and be extended even further along both banks of the river on July 26, in the final hours before the evening ceremony.

Anyone who wants to enter the zone in the eight days before the ceremony and on July 26 itself will need to pre-register online and will “systematically” be subjected to the background security checks known in France as an “administrative investigation,” Nunez said.

Those affected will include people who work and live inside the perimeter, as well as their guests, and people going to hotels or restaurants inside the zone, he said.

Paris Mayor Anne Hidalgo said about 20,000 people live and work inside the perimeter. In the days before the ceremony, there will be places to cross the river and ways to visit the Louvre and other museums along the river without having to enter the security zone.

Nunez said the security check for those affected won’t include searching through their communications. “It’s not as intrusive as that,” he said.

“We verify if the person is known in a certain number of intelligence files,” he said.

Anyone flagged by the checks could be barred from the zone. “It will obviously be case by case,” Nunez said.

Ticket-holders for the ceremony won’t need to pre-register on the online platform that will open May 10. But Nunez said French intelligence services can do checks on them, too.

Separately, the top administrative official for the Paris region, Prefect Marc Guillaume, pledged that tests on the water quality in the Seine will be made public, without specifying when or how regularly.

Marathon swimmers and Olympic and Paralympic triathletes are scheduled to race in the river, which is being cleaned up for the Games. From July 1, samples will be collected from 36 spots on the river and its tributary, the Marne, Guillaume said.

“We will be totally transparent about our results,” he said.

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