Vickers stepped down as head of US-CERT on Friday, according to an email sent to employees by Roberta Stempfley, acting assistant secretary for cybersecurity and communications at the Department of Homeland Security (DHS), and obtained by Reuters news service.
Stempfley did not disclose the reason for Vickers' resignation, and a DHS official declined to comment. Lee Rock, who was deputy director of US-CERT, will serve as acting director until a replacement is found.
The resignation follows high-profile attacks against the CIA, the Senate, and other government sites.
In June, the hacker group Lulz Security claimed to have hacked into the public websites of the CIA and the Senate. In addition, the group claimed to have hacked into the site of the broadcaster PBS, after it aired a documentary Lulz considered critical of WikiLeaks founder Julian Assange.