The Drug Enforcement Administration paid confidential informants millions of dollars without appropriate oversight and used sources in ways that might run afoul of the Constitution, the Justice Department inspector general found.

Those findings were part of a withering 65-page report released Thursday that outlined serious missteps in the DEA's use of confidential informants and that recommended that the agency substantially tighten its policies and procedures.

Deficient oversight led to instances of fraud and abuse, Inspector General Michael Horowitz found. In one, according to the inspector general's report, the DEA paid more than $469,000 over a five-year period to a source who had previously been deactivated for lying in trials and depositions. In another, the DEA paid an Amtrak employee hundreds of thousands of dollars "for information that was available at no cost to the government," the inspector general found.

The DEA wrote in its official response to the report that it had already made many improvements in agents' use of confidential informants and would work to make even more. A DEA spokesman did not immediately return a phone message seeking comment.

Using confidential informants to uncover illegal drug activity is a critical part of DEA agents' work. According to the inspector general's report, the agency had more than 18,000 active informants between October 2010 and September 2015, and more than 9,000 of those received about $237 million in payments.

The sources ranged from criminals providing information on their associates to airline, train and parcel-service employees providing tips on suspected drug traffickers' movements.

The work, for all involved, can be lucrative. The inspector general found one airline employee who received more than $600,000 in less than four years, and a parcel employee who received more than $1 million in five years.

The inspector general found, though, that more money did not always come with more oversight. The DEA's Intelligence Division, for example, paid $25 million to eight sources over a five-year period and "did not independently validate the credibility of these sources, or the accuracy of the information they provide," according to the inspector general. The Intelligence Division, according to the report, generally relied on "DEA field offices' risk assessments and determinations that confidential sources are reliable."

The DEA also paid some sources enough that they could recruit subcontractors and had "no controls, policies, or procedures for interactions with these 'sub sources,' " the inspector general found.

The inspector general found that sources who had been deactivated, sometimes because of unsatisfactory behavior, continued to take DEA money. According to the report, the agency paid about $9.4 million to more than 800 previously deactivated sources between fiscal 2011 and 2015.

The reasons for the deactivations were not always clear, but the inspector general found two examples that "raise serious questions of propriety."

In one, according to the inspector general's report, the DEA paid more than $962,000 to the Amtrak employee who was the subject of an inspector general investigation. More than $44,000 of that money came after the source was deactivated, according to the inspector general's report.

In the other instance, the DEA, with the Justice Department's approval, reactivated a source who had been shut down for lying in trials and depositions.

Over the next five years, the inspector general found, 13 DEA field offices paid the source more than $400,000. The source then lied again, this time to a prosecutor during an investigation, and was deactivated a second time. But, according to the inspector general's report, the DEA kept paying the source, who would eventually make more than $469,000.

The inspector general also criticized how the DEA used transportation and parcel service employees in ways that pushed the boundaries of agency guidelines and the Constitution. Instead of using such employees as tipsters, DEA agents would request their sources search proprietary databases, directly transfer packages to the DEA, or turn over suspicious travel itineraries. In some cases, according to the inspector general, agents would ask for entire passenger manifests almost daily. Aside from Amtrak, no company or airline is named in the report.

From fiscal 2011 to fiscal 2015, the DEA used at least 33 Amtrak employees and eight TSA employees as sources, paying the Amtrak employees over $1.5 million and the TSA employees over $94,000, according to the report.

The use of TSA was particularly troublesome, as TSA screeners "are obligated to report to law enforcement suspected criminal activity that they observe in the course of their duties," the inspector general found. And the amount of money at stake and hands-on guidance that DEA agents offered raised other questions of impropriety.

"Together, these factors call into question whether the source is truly providing information independently or is acting as DEA's agent, the latter of which could have implications relating to compliance with the Fourth Amendment's protections against unreasonable searches and seizures," the inspector general wrote.