BRUSSELS

Arrests in connection with a blast outside the US Embassy in Oslo have risen to four, Norwegian authorities said on Friday.

The latest detainee includes the mother of three brothers, all of whom were earlier arrested and charged with "terrorist bombing," according to broadcaster NRK.

One of the brothers has admitted placing a homemade explosive device at the embassy entrance early Sunday morning, while the others deny the charges and have requested release.

Police held the detention hearings behind closed doors, citing concerns that media coverage could interfere with the ongoing investigation or influence evidence collection.

The suspects are Norwegian citizens of Iraqi origin and were not previously known to authorities.

Police said the investigation is ongoing to determine possible motives, including whether the attack is connected to geopolitical tensions in the Middle East.

No injuries were reported in the explosion, but the device caused damage to the embassy entrance.

The suspects will remain in custody as the investigation continues.

Local media reported that the blast occurred at around 1 am on Sunday, and authorities quickly launched a terrorism investigation.