After the club's relegation from the Bundesliga, angry fans assaulted the team bus, prompting Schalke 04 to request police security at training next week.
On their return to Gelsenkirchen early Wednesday, a large group of enraged fans pelted the team bus with eggs and threatened players after Schalke was relegated after 30 years in Germany's top flight following a loss at Bielefeld.
This season, the team has only won two of its 30 games.
Following Tuesday's loss, which was their 21st league defeat of the 2020/21 season, some players were reportedly followed to their homes and attacked.
Until police intervened, at least one player's car was said to have been severely damaged.
While Schalke do not have a game this weekend, sports director Peter Knaebel has requested police assistance when training resumes on Monday.
In a statement Friday, Schalke said they have "already initiated measures to ensure the safety and security of the players and staff".
The players and staff came together today for the first time since the horrific scenes on Wednesday (21/4).— FC Schalke 04 (@s04_en) April 23, 2021
On behalf of the club, Peter Knäbel apologised to all those affected. ⤵️
Knaebel also told Bild that he "can live with it" if any player refuses to play their last four remaining games.
The Schalke boss blames himself for not taking steps to protect the team on their return from Bielefeld.
"People were in danger, things got damaged. We were lucky that nothing worse happened to our people," Knaebel told German daily Bild.
"I apologize to the players and staff."
In a statement, the club said it will not tolerate instances where the "health and safety of its employees are threatened or endangered."
"A line was crossed on Wednesday morning, and these events have deeply shaken the club.
"Violence is never justified, neither in sport, nor in society."