The investigations have included employees of the National Security Agency, the National Reconnaissance Office, and the Defense Advanced Research Projects Agency — which deal with some of the most sensitive work in intelligence and defense — among other organizations within the Defense Department.

The number of offenders is a small percentage of the thousands of people working for sensitive Pentagon-related agencies. But the fact that offenders include people with access to government secrets puts national security agencies “at risk of blackmail, bribery, and threats, especially since these individuals typically have access to military installations,’’ according to one report by the Defense Criminal Investigative Service from late 2009.

Some of the individuals have been prosecuted and other cases have been dropped, while more have languished several years without resolution, according to the previously undisclosed documents about the investigations.

The more than 50 pages, compiled by the investigative service, part of the Pentagon’s Inspector General’s Office, contain summaries of investigations initiated since 2002, including some cases that remain open.

The uneven discipline reflects difficulties in bringing prosecutions, according to specialists. The evidentiary standards are high for prosecution in child pornography cases, according to child welfare specialists, including positively identifying victims as underage or known victims of abuse. In others, evidence was lost or misplaced and investigators said they lacked sufficient resources to complete all of them.

Gary Comerford, a spokesman for the Pentagon’s Inspector General, said the agency takes such cases very seriously but said he could not comment on individual investigations.

Many of those apprehended were swept up in a much broader probe initiated by the Immigration and Customs Enforcement agency in 2006. Operation Flicker identified an estimated 5,000 people who had paid money over the Internet to access websites operated overseas. But until now, it has not been disclosed that a sizable number of cases were referred to the Defense Department for investigation because they involved military personnel, intelligence officials, or defense contractors.

The investigative documents were provided to the Globe by a government official after they were approved for public release.

The exact numbers of cases involving Defense Department personnel were not contained in the reports and officials at DCIS could not immediately provide statistics. But the official reports indicate that more than 30 government employees were investigated.

Purchasing child pornography is a crime; accessing it on a government computer is also a violation of laws governing the misuse of government property.

At least two of the cases were contractors with top secret clearances at the National Security Agency, which eavesdrops on foreign communications, according to the documents. When one of the contractors was indicted two years ago, he fled the country and is believed to be hiding in Libya, according to a summary of the investigation from last year. The other was sentenced in 2008 to more than five years in prison and lifetime probation.

A separate case involves a contractor working at the National Reconnaissance Office, the agency that builds and operates the nation’s spy satellites. The individual admitted in 2008 when he was being interviewed to renew his security clearance that he viewed child pornography at least twice a week on his home computer.

As of December, the individual had been transferred to an agency field office in New Mexico and had not been charged. A National Reconnaissance Office spokesman, Rick Oborn, said he was aware of a few cases of agency employees accessing such images but could not immediately say whether the particular contractor was still working for the organization.

Specialists in child protection expressed alarm at the revelations, but said it was not that surprising to find even officials in sensitive government positions engaging in such activity.

“Some are in high-ranking positions, in positions of trust,’’ said John Sheehan, executive director of the exploited child division at the National Center for Missing and Exploited Children, which has been consulted on many of these cases and has reviewed 36 million images of alleged child pornography since 2002 at the request of law enforcement agencies. “There isn’t a profile or stereotype, which makes it even more challenging for law enforcement.’’

The Pentagon’s investigation reports show that personnel found frequenting the illegal websites worked at a variety of Pentagon installations.

Thirteen suspects were identified in California, including individuals who worked at some of the most sensitive military installations on the West Coast. One was a contractor at Edwards Air Force Base, where weapons testing is conducted, while another worked at the Naval Air Warfare Center at China Lake.

Their positions gave the cases priority at the immigration and customs agency that first uncovered them, according to the reports, “because the subjects are DoD employees who possess security clearances.’’

A large amount of pornography was found on the office computer of a program manager at the Defense Advanced Research Projects Agency, including images that appeared to be of children. DARPA is responsible for developing some of the military’s most secret weapons and technologies. Charges were not pursued because there were no images of known victims of abuse, something that is routinely needed to bring charges, the case summary said.

A DARPA spokesman, Eric Mazzacone, declined to comment.

Other agencies where personnel were investigated are blacked out in the documents because their jobs were so highly sensitive, including a case from 2007 in which a national security official had 93 documents, 8,400 pictures, and 200 movies “that were evidence of receipt of child pornography.’’ The individual was sentenced to five years in prison and five years of supervised release.

Others have not led to criminal prosecutions, such as the 2007 case involving an employee at the Defense Contract Management Agency in Hartford who had about 40 images believed to constitute child pornography on a government-issued computer. The individual was not prosecuted because the ages of the individuals depicted in the images could not be determined or positively identified as known child victims, according to the reports.

Another case opened in August 2007 involving a Defense Department contractor was closed “due to a lack of resources,’’ a November 2009 report from Pentagon criminal investigators said.

The case was referred back to Immigration and Customs Enforcement.

Bryan Bender can be reached at bender@globe.com.