On a day when families are grieving for the loss of loved ones, including children, after the attack in Manchester in which 22 people were killed, it might seem perverse that cooperating to catch criminals and terrorists would be at all controversial.

Yet intelligence sharing remains one of the most delicate aspects of counterterrorism. Officials stress that after the Paris and Brussels attacks cooperation has improved, but there is still a reluctance to share information at the European level.

Britain’s exit from the European Union complicates matters even further.

Speaking at a security conference in Munich in February, EU Home Affairs Commissioner Dimitris Avramopoulos complained that “we are still not sharing enough when it comes to information and intelligence.” Many countries, including Britain, still prefer to share sensitive information bilaterally because of fears about where the information could end up. “It could be counterproductive to share some intel because it could endanger the source,” said an EU diplomat from Northern Europe.

Even before Monday’s horrific attack on concert-goers at the Manchester Arena, intelligence sharing was set to be a difficult topic in Brexit negotiations.

Both sides want to continue cooperating — indeed, Theresa May has made maintaining defense and security cooperation a priority.

But things got off on the wrong foot in March when many in Brussels were irked by May’s tone on security in her letter to the EU triggering Article 50, the start of formal divorce proceedings.

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“In security terms a failure to reach agreement would mean our cooperation in the fight against crime and terrorism would be weakened,” she wrote. To many that sounded like a veiled threat: Either we reach a deal on Brexit or the EU will not be able to access British intelligence and security assets — considered among the best in Europe.

The EU27’s position became clearer Monday with the release of the final text of Brexit negotiator Michel Barnier’s mandate for the talks.

The paragraph that sets out the EU27’s position on law enforcement cooperation includes a more explicit statement than earlier drafts on the bloc’s desired rules for the use of intelligence and other data. It was amended to read that arrangements should comprise rules on the protection of personal data and “classified information, including security data.” In other words, the EU27 wants, as part of the Brexit negotiations, to formalize the sharing of intelligence and security cooperation with the U.K.

With the identity and motives of the Manchester attacker still unknown, it is too early to say whether more security cooperation in Europe could have prevented that attack. However, with the terror threat still very present across the Continent, it will serve as a painful reminder of the consequences of failing to stop terrorists before they execute their plans.

This insight is from POLITICO‘s Brexit Files newsletter, a daily afternoon digest of the best coverage and analysis of Britain’s decision to leave the EU. Read today’s edition or subscribe here.