European investigators tracking a terrorist suspect linked to to the Islamic State say there were close to nabbing him until their spyware on his phone shut off.

It turns out that WhatsApp, which is owned by Facebook, had informed 1,400 users - including the suspect - that their phones had been had been hacked by an 'advanced cyber actor,' reported The Wall Street Journal.

The investigators say they had following the suspect using spyware software from NSO Group, an Israeli company.

WhatsApp argues that such means of spying by the government is illegal and that it was protecting its users. 

According to The Journal, law enforcement had been tracking the suspect because they feared they would be a terrorist attack during the holidays. 

An unidentified judge had granted them permission to use whatever means necessary to get into the suspect's phone.

When authorities learned he was using WhatsApp's encrypted messaging, they knew they needed spyware.

The government of the Western European country had a contract with NSO that authorities used to hack suspects.