WASHINGTON — The Pentagon’s investigation of defense and intelligence employees who downloaded child pornography is being criticized in Congress after the Department of Defense acknowledged that its investigators failed to check thoroughly whether its employees were on a list of suspected porn viewers.

In 2006, the Immigration and Customs Enforcement agency, which conducts Internet pornography investigations, produced a list of 5,200 Pentagon employees suspected of viewing child pornography and asked the Pentagon to review it. But the Pentagon checked only about two-thirds of the names, unearthing roughly 300 defense and intelligence employees who allegedly had viewed child pornography on their work or home computers.

The defense investigators failed to check an additional 1,700 names on the list, defense officials have revealed in correspondence with Senator Charles Grassley, Republican of Iowa.

Downloading child pornography is a federal crime, punishable by prison sentences of five to 20 years. But members of Congress and other officials say it raises additional concerns when such materials are accessed by employees with high-level security classifications, because it leaves them vulnerable to blackmail.

Acknowledging the lapse, the Pentagon has told Grassley that child porn investigations were not a high priority at the time of the Immigration and Customs Enforcement investigation, and that it is now checking the additional names. A spokesman would not elaborate in response to a question from the Globe, which first reported the child pornography investigation last summer.

“These cases were not considered a priority by the Defense Department in the first place, and they should have been,’’ Grassley said this week in an interview. Grassley wrote last month to the Department of Defense inspector general, Gordon S. Heddell, demanding a thorough accounting of the department’s actions.

“We want a change in behavior in the Defense Department where things of this criminal nature are a top priority, even more than government employees at other agencies because of the national security connections,’’ Grassley said.

The cases stemmed from an expansive ICE investigation four years ago known as Project Flicker. The agency, which is part of the Department of Homeland Security, conducted the inquiry because the illegal content was downloaded from foreign-based websites, giving ICE jurisdiction under its customs mandate.

The new information uncovered by Grassley and his staff suggests the number of violations linked to the Pentagon could be higher than the 300 previously reported. No new count has been released since investigators began going over their lists again.

In a Nov. 19 letter from Heddell to Grassley, the Pentagon inspector general said that child pornography was “not one of DCIS’ investigative priorities.’’