The bulletin did not cite any specific threat but said that the risk of violence will persist for weeks. It warned that some extremists may be "emboldened" by the Jan. 6 attack on the U.S. Capitol.
The company also said it has banned "numerous individuals" that it found to be associated with hate groups or involved in criminal activity at the Capitol.
The violent insurrection at the U.S. Capitol stunned many in Germany, especially those who grew up in the American sector of then-West Berlin.
In remarks at the Vatican on Sunday, Pope Francis condemned the violence at the U.S. Capitol and urged Americans to come together for the "common good."
The U.S. is reporting more than 271,000 new cases each day. Congress' attending physician says lawmakers who sheltered in place last week may have been exposed to the virus.
Wednesday's violent insurrection at the Capitol led to the deaths of five people. Some lawmakers and others worry that it was just the beginning.
In an interview with NPR, the senator called the president's conduct a "flagrant dereliction of his duty." He also criticized Missouri Sen. Josh Hawley for his challenge to the election's results.
Simon & Schuster says it has decided not to publish Hawley's forthcoming book The Tyranny of Big Tech, suggesting that the lawmaker helped foment Wednesday's violence.
The Capitol Police said officer Brian D. Sicknick died from his injuries and that several other officers injured in the attack remain hospitalized.
A mob stormed the U.S. Capitol after President Trump urged supporters to march to the building to oppose the election results. Roughly 14 hours later, Congress affirmed Joe Biden's victory.