Cohen, 57, once sponsored a bill creating a computer hotline to report child pornography and other Internet crimes.
Cohen, of Roselle, served in the Assembly from 1990 to 1991 and from 1994 to present. He had been serving as deputy speaker.
According to two other officials with knowledge of the investigation, who spoke on condition of anonymity because they were not authorized to discuss it, staffers found the pornography on Cohen's computer.
They told state legislative staff, who in turn reported it to the Attorney General's Office, which seized computers from Cohen on Wednesday. He has not been charged, the officials said.
They said Cohen has since been hospitalized for psychiatric treatment.
The investigation was first reported last Thursday on the Web site of The Record of Bergen County.
Governor Jon Corzine issued the following statement regarding the resignation of Neil Cohen from the Legislature:
"The allegations made concerning former Assemblyman Cohen are deeply disturbing. All of us who have worked with him are shocked by the reports. Child pornography reflects a horrible debasement of children that is totally intolerable. Creating, distributing and using child pornography should be pursued vigorously by law enforcement wherever it is found."
