It was early morning on Oct. 7, 2023, and Hamas fighters had just breached the Israeli border, when Ahmed Omer, a young Sudanese man with an aptitude for computers, launched a different kind of attack.

Sitting at a computer, he mounted a long-distance cyberassault on the online early warning systems used in Israel to alert citizens to danger. The systems were briefly disabled, preventing potentially lifesaving warnings from reaching Israelis about Hamas’s deadly assault, in which about 1,200 people were killed and more than 200 abducted.

That was the case made against Mr. Omer and his brother, Alaa Omer, in a criminal indictment unsealed in California this week. “This was the most dangerous cyber group in terms of DDoS attacks in the world,” E. Martin Estrada, the United States Attorney for the Central District of California, said in a call with reporters.

The brothers are accused of running a group called Anonymous Sudan, which for the year it existed launched as many as 35,000 cyberassaults known as distributed denial of service, or DDoS, attacks. The attacks disrupted websites belonging to government agencies, including the F.B.I. and Justice Department, and to news agencies, such as The Washington Post and CNN, according to the indictment.

They also attacked hospitals in various countries.

The indictment outlines in detail actions against the United States, Israel and a number of other countries, including Denmark, France and Sweden. The California attorney’s office claims jurisdiction because the operation attacked American entities.

In February, the brothers shut down critical computer systems belonging to the Cedars-Sinai hospital in Los Angeles, causing emergency services to temporarily divert patients to other hospitals, according to the indictment. The brothers, who have been arrested and are in custody in an unspecified country, claimed at the time that the attack was in retaliation for Israel’s bombing of hospitals in Gaza.