Responding to an Associated Press investigation, three U.S. senators on Tuesday urged Defense Secretary Ashton Carter to lift what they called the military justice system's "cloak of secrecy" and make records from sex crimes cases readily accessible.

In a letter to Carter, Democrats Barbara Boxer of California, Kirsten Gillibrand of New York, and Mazie Hirono of Hawaii say the lack of transparency in military legal proceedings "calls into question the integrity of the institution and hides the system's shortcomings."

AP's investigation, published last month, found more inmates are in military prisons for sex crimes against children than for any other offense. But the military's opaque justice system prevents the public from knowing the full scope of their crimes or how much time they actually spend behind bars.

The letter underscores the dissatisfaction among a small but vocal group of lawmakers with the Defense Department's efforts to stamp out sexual assaults within the ranks and punish offenders. The senators do not threaten legislative action if Carter doesn't take the steps they recommend, yet that remains an option. Boxer, Gillibrand and Hirono all have previously advocated more aggressive changes in military law and policy to prevent sexual assaults.

The senators said Carter should begin to restore the public's confidence in military justice by applying the same standards of openness that civilian courts use. This includes making trial records and documents from appellate cases readily available through the online Public Access to Court Electronic Records service, known as PACER. The military does not have a comparable repository.

Most records from military sex crimes cases and other offenses are available only through the Freedom of Information Act and it can take multiple requests and months of waiting to get a response. At the time AP's investigation was published, it had filed 17 separate requests since May under the open-records law for documents from more than 200 military sexual assault cases that ended with convictions. The military services had provided complete trial records for five cases and partial records for more than 70 others.

"We find it troubling that the military delayed releasing -- or even completely withheld -- critical facts about these cases," the senators wrote. "The obscurity and sluggish response time only creates suspicion and mistrust."

Boxer, Gillibrand and Hirono also said the Defense Department's procedures for tracking sexual assault cases may result in an underestimation of the problem. Reports of sexual assaults against children are not included in annual reports of sex crimes the department sends to Congress, they wrote. That report is focused primarily on adult-on-adult offenses.

"Taken in combination with the AP's findings, we are worried that we continue to lack a clear picture of the full rates of sexual assault in the military," the senators said.

They also described as "incredibly alarming" the military's use of pretrial agreements in child sex crimes cases that greatly reduced the amount of prison time for convicted service members.

"While there may have been legitimate reasons for some of these decisions, it is prohibitively difficult to independently assess the decisions in a timely manner since the military services do not include legal records or trial outcomes in an online database like the civilian court systems," the senators wrote.