In 2015 and 2016, terrorists linked to al-Qaida and the Islamic State unleashed a series of deadly attacks in Europe that killed nearly 200 people and injured hundreds more.

ProPublica and Frontline have interviewed some of Europe’s top counter-terror officials, several of whom said the terror campaign has exposed a number of longstanding, well-known failings in Europe’s defenses.

In remarkably candid interviews with Sebastian Rotella, ProPublica senior reporter, officials acknowledged that most of the leading plotters in the attacks were on the radar of European intelligence services. Others escaped detection because Europe has failed to put in place effective intelligence sharing and border enforcement, such as procedures for tracking air travelers that became standard in the United States after the 9/11 attacks.

“The institutional flaws in the European system are multiple,” said Jean-Louis Bruguiere, a former judge in France who has led some of his country’s most high-profile terror investigations. “I don’t know what we are waiting for.”

Bruguiere said the reluctance to tighten security threatens the existence of the 28-nation European Union, which has abolished internal borders. “If European policy is unresponsive,” Bruguiere said, “We will be putting up barriers again. Nobody wants it, but we will not be able to do otherwise if we are incapable of protecting ourselves.”

“Terror in Europe” premieres Tuesday, Oct. 18 at 10 p.m. EST/9 p.m. CST on PBS stations and online at pbs.org/frontline.