Spanish terror police make an arrest | Angela Rios/AFP via Getty Images Spain arrests four over suspected ISIL people smuggling links Those detained are believed to have had contact with at least one alleged terrorist arrested in Salzburg, last December.

Spanish authorities arrested four people Monday over their alleged involved in an irregular migration network, thought to have been used by the ISIL terror group to move its militants to Europe.

According to an interior ministry statement, all four had links to the network operating along the "Syrian refuge route" between Turkey and Eastern Europe that was allegedly used to bring terrorists to Europe, including those involved in last November's Paris attacks.

The ministry said the four suspects had contact with at least one alleged terrorist arrested in Salzburg, Austria, last December.

Two of the suspects were arrested in Galicia in northwestern Spain and two others in Almería in the southeast of the country. Authorities did not reveal their identities.

Since raising the security threat alert to its second-highest level in 2015, the ministry said authorities have arrested 168 people suspected of involvement in terrorist activity.