PARIS—As police intelligence agents in Paris tracked Islamist radicals over the summer, one of their longtime colleagues was quietly embracing a more extreme version of his Muslim faith.
While away from the office, Mickaël Harpon, a 45-year-old convert to Islam, had begun dressing in more religious garb and established ties with followers of Salafism, a fundamentalist strain of Islam, according to prosecutors. At work, he appeared to avoid physical contact with women, colleagues would later tell prosecutors.
During his lunch break last week, Harpon bought two knives, returned to the office and stabbed four of his colleagues to death. He wounded a fifth one before a rookie police officer shot him dead.
Harpon’s drift toward fundamentalist Islam went undetected even though he worked inside the Paris police’s intelligence division: an elite unit charged with monitoring Islamist extremists across the French capital.
As one of the unit’s information technology specialists, Harpon sat at a desk located just steps away from the division’s leadership. He held a high-level security clearance and had access to the unit’s computer system that contains detailed information about undercover police operations under way in mosques across Paris.
The attack has stunned France and its sprawling counterterrorism apparatus, shattering confidence in the security-services’ procedures for detecting extremists in their midst. Investigators are examining whether Harpon compromised ongoing operations and revealed the identity of undercover police agents.
Authorities discovered several USB flash drives at his desk, one containing the personal information of agents and violent Islamist propaganda, authorities said. Some 160 agents are poring over the data on that flash drive, officials said.
A key question is whether Harpon downloaded that data onto the flash drive for his job as an information technology assistant in the division—which monitors online Islamist propaganda—or to send it to his extremist contacts that could use it to target the police.
Since the attack, several police stations in the Paris region have received threatening calls in which officers could hear Islamist songs glorifying Islamic State, authorities said.
The information that has emerged from the investigation so far portrays Harpon as a capable employee who was growing more rigorous in his faith and struggling with a disability that he believed was short-circuiting his career in the police.
Born in the French Caribbean island of Martinique, Harpon suffered a bout of meningitis in his youth, which damaged his hearing and required him to wear a hearing aid. He was hired by the intelligence division of the Paris police in 2003. He was dedicated and efficient, police officers say, and always ready to help.
Harpon’s job took him to the heart of Paris, where he walked the wide halls of the city’s police headquarters, housed in a massive 19th-century stone barracks facing Notre Dame Cathedral. One of France’s oldest security services—its origins date to the 17th century—the Paris police is sometimes called a state within a state, with its own intelligence service that tracks extremist movements across the French capital.
The independence of its intelligence division came with a price, often putting it into conflict with France’s main domestic security agency, the DGSI, which has nationwide jurisdiction.
“They didn’t share much,” said Bernard Squarcini, who was France’s domestic intelligence chief from 2008 through 2012. “They considered themselves to be autonomous.”
“Their security measures are not as strong as the other services,” Mr. Squarcini added.
A spokeswoman for the Interior Ministry said the Paris police intelligence unit had started working more closely with the DGSI since Mr. Squarcini left. She added that comparing the unit’s internal security procedures with those of the DGSI didn’t make sense given the DGSI’s broader mission, which includes counterespionage.
Harpon converted to Islam several years after joining the police, around the time he was living with his companion, a Muslim woman from Madagascar.
In 2008, she filed a complaint against him for assault. Though she later withdrew it, Harpon received an administrative sanction from the police over the accusation.
Six years later, Harpon married his companion, a step that should have prompted another background check to maintain his security clearance, French Interior Minister Christophe Castaner said.
“There wasn’t a check,” Mr. Castaner said. “Would that have changed things? I don’t know. But that’s a malfunction.”
In 2015, Harpon’s division came under scrutiny after Islamist militants stormed the offices of the satirical magazine Charlie Hebdo and a kosher supermarket, leaving 17 dead across the Paris region. The unit had been following one of the militants in the months before the attacks but stopped when he moved away from Paris, advising the DGSI that the surveillance would end.
Months after the attack, one of Harpon’s colleagues in the intelligence division heard him say about the victims at Charlie Hebdo: “They deserved it.”
The employee reported the comment to his superiors, but officials neither placed a mention of it in Harpon’s personnel file nor conducted another background check on him. His next background check to maintain his security clearance was slated for 2020.
“A serious malfunction,” Mr. Castaner said.
In recent years, Harpon went every morning before 5 a.m. to pray at a mosque near his house in Gonesse, a working-class northern suburb of Paris, according to Abdelaziz, a man who describes himself as a friend in an interview posted by the mosque’s imam on his YouTube channel. Gonesse is outside the jurisdiction of the Paris police and its intelligence division.
Harpon never showed signs of radicalization, but recently expressed frustration about his work, said Abdelaziz, who didn’t give his last name.
“He felt people didn’t take him seriously because of his handicap,” Abdelaziz said.
The night before the attack, neighbors reported they heard him shout out prayers.
The next morning, Harpon and his wife exchanged 33 text messages, according to officials. His last one said, “Allahu akbar”—God is great in Arabic—“follow closely our beloved Prophet Muhammad and study the Quran.”
His wife told investigators she thought he was going to kill himself.
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