In a report to be published on Monday, and provided in advance to The New York Times, Kaspersky Lab says that the scope of this attack on more than 100 banks and other financial institutions in 30 nations could make it one of the largest bank thefts ever — and one conducted without the usual signs of robbery. The Moscow-based firm says that because of nondisclosure agreements with the banks that were hit, it cannot name them. Officials at the White House and the F.B.I. have been briefed on the findings, but say that it will take time to confirm...