Six arrests have been made in a terrorism investigation into youths who have travelled or tried to travel to Syria to fight with militants, including the Islamic State group, federal authorities said.

A spokesman for the US attorney’s office said the arrests were made on Sunday in Minneapolis and San Diego and there was no threat to public safety. Spokesman Ben Petok did not give details about the charges.

The attorney’s office and the FBI planned a news conference on Monday to announce details.

Kyle Loven, spokesman for the Minneapolis office of the FBI, said six people were arrested but gave no further details. An FBI spokesman in San Diego referred questions to Loven.

Authorities said a handful of Minnesota residents have gone to Syria to fight with militants within the last year. At least one has died while fighting for the Islamic State group.

Since 2007, more than 22 young Somali men have also travelled from Minnesota to Somalia to join the terrorist group al-Shabab.

Four Minnesotans have already been charged in connection with supporting terror groups in Syria, including the Islamic State group.

One man, 19-year-old Hamza Ahmed, was stopped at a New York airport in November as he and three others were attempting to travel to Syria. Ahmed has been indicted for lying to the FBI during a terrorism investigation, conspiring to provide material support to the Islamic State group, and attempting to provide material support. He has pleaded not guilty.

But there have been no public charges filed against his three companions, and little information had been released about them. An FBI affidavit said they are all between the ages of 19 and 20.