A number of United States banks, including JPMorgan Chase and at least four others, were struck by hackers in a series of coordinated attacks this month, according to four people briefed on a continuing investigation into the crimes.

The hackers infiltrated the networks of the banks, siphoning off gigabytes of data, including checking and savings account information, in what security experts described as a sophisticated cyberattack.

The motivation and origin of the attacks are not yet clear, according to investigators. The F.B.I. is involved in the investigation, and in the past few weeks a number of security firms have been brought in to conduct forensic studies of the penetrated computer networks.

According to two other people briefed on the matter, hackers infiltrated the computer networks of some banks and stole checking and savings account information from clients. It was not clear whether the attacks were financially motivated, or if they were collecting intelligence as part of an espionage effort. Aside from JPMorgan, it was also not immediately clear which other banks were infiltrated.