PARIS (Reuters) - A police employee who fatally stabbed four colleagues in the force’s Paris headquarters ran an online search for “how to kill infidels” an hour before he rampaged through the building, a judicial source said on Wednesday.

Data on the cell phone of Mickael Harpon, a 45-year-old IT worker who converted to Islam a decade before last October’s attack, supported suspicions his motive was terrorism-related, the judicial source and a police source told Reuters.

“His phone contained a lot of data but we have already isolated one internet search carried out an hour before the attack: ‘how to kill infidels’,” the judicial source said, confirming a report in daily Le Parisien.

Harpon was shot dead by police at the scene after he killed three officers and one administrative worker with a kitchen knife.

The investigation into the attack is being led by anti-terrorism prosecutors, which usually indicates a possible terrorism link is the focus of inquiries.

The judicial source said it took several months to break the codes Harpon had used to secure his personal smartphone.

Both sources said the data recovered on it appeared to confirm suspicions Harpon had been radicalised.

The police source, however, cautioned that no evidence had yet been found that Harpon had links to militant group Islamic State, which claimed the attack, or that he had planned it far in advance. The data analysis was still ongoing.

Harpon used the Arabic word “kouffar” (infidels) in some of his Internet searches, according to Le Parisien.

An internal report released by French authorities after the attack said that, in 2015, two of Harpon’s co-workers had complained about his reaction to the killing by Islamist gunmen of 12 staff at French satirical magazine Charlie Hebdo.

The complaint never went beyond the boss of Harpon’s department, who concluded that he posed no threat.

This led to a review of security services practices and in January the government adopted enhanced screening measures. Several policemen suspected of Islamist sympathies have been removed from their positions since Harpon’s attack, the Paris police chief said around a month after it happened.