The Pentagon is promising to expand its crackdown on improper travel payments after an internal investigation found widespread errors in vouchers that troops submit for reimbursement, according to a new report.

The Defense Department Inspector General released a report Thursday showing that the percentage of inaccurate payments made under the Defense Department Travel Pay program grew from 5 percent in 2012 to 7 percent for 2014, when improper payments totaled more than $458 million. A majority were over payments, though in some cases troops were also shortchanged. It's unclear how much of this money the Defense Department ultimately recoups.

In many cases, officials approved travel expenses for airfare, hotels and rental cars despite service members' failure to submit receipts, according to the report.

The uptick came despite the Pentagon's force-wide effort to reduce improper payments and comply with a 2010 federal law designed to reduce wasteful spending and over payments for official travel.

The IG noted that the 2013 program to reduce improper payments did not include any effort to identify why improper payments were approved. Instead, the remediation program focused on new training for travelers and the officials who approve travel expenses.

"Without identifying the reasons that authorizing officials approve improper vouchers for payment, DoD will continue to be at high risk for making improper payments for travel," according to the IG report.

In response to the investigation, the Defense Department promised to begin requiring the military services to identify the root causes for why officials continue to approve improper payments.

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Military officials also agreed to develop ways to fix those root causes, saying also that the department will require the Defense Finance and Accounting Service to provide the services with quarterly reports outlining the errors found in travel vouchers submitted by troops and civilians.

Some 51 percent of the improper payments involved per diem payments. But the Pentagon's budget office did not offer any reasons for why per diem errors occurred, according to the IG report.

A review of about 5,000 random military travel vouchers filed during the latter half of 2014 found all of the vouchers had at least one error, according to the report.