Egypt's state security prosecution has said that national security investigations revealed that independent news website Mada Masr was founded by the now-outlawed Muslim Brotherhood group with the aim of spreading false news.

The announcement came after plain clothes security forces raided the website's office in Cairo on Sunday, arresting three staff members including editor-in-chief Lina Attalah.

This followed the arrest of one of their editors, Shady Zalat, at his home a day earlier and detaining him in an unknown location. The four were released later on Sunday.

The high state security prosecution had authorised the inspection of Mada Masr's office, a statement from the prosecution office said late on Monday. This followed "national security investigations which concluded that the website was founded by the Brotherhood group to spread false news and rumours with the aim of disrupting public order," it added.

"The inspection resulted in the seizure of equipment used in the crime," the statement added.

Mada said on Facebook on the day of the arrests that nine plain-clothed officers questioned the journalists on site for several hours and asked some of them to unlock their phones and laptops, before the three staff members were arrested.

Mada Masr was one of hundreds of news websites blocked by Egyptian authorities two year ago, but the independent outlet has continued to publish critical and investigative reports in Arabic and English, including via its Facebook page to challenge the ban.

The website can be accessed locally only through a virtual private network (VPN) app.