The Obama administration knowingly let in at least 16 admitted MS-13 gang members who arrived at the U.S. as illegal immigrant teenagers in 2014, a top senator said Wednesday, citing internal documents that showed the teens were shipped to juvenile homes throughout the country.

Sen. Ron Johnson, chairman of the Senate Homeland Security Committee, said a whistleblower turned over Customs and Border Protection documents from 2014 detailing the 16 people who were caught crossing the border.

“CBP apprehended them, knew they were MS-13 gang members, and they processed and disbursed them into our communities,” Mr. Johnson, Wisconsin Republican, said.

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The gang members were part of the surge of UAC, or “unaccompanied alien children,” as the government labels them, who overwhelmed the Obama administration in 2014, leaving Homeland Security struggling to staunch the flow from Central America.

Officials at the time said the children should be treated as refugees fleeing horrific conditions back home — though security analysts said the children were prime recruiting territory for gangs already in the U.S.

Mr. Johnson said the image of UAC as little children is misleading. Out of nearly 200,000 UAC apprehended between from 2012 to 2016, 68 percent were ages 15, 16 or 17 — meaning older teens. The majority were also male, making them targets for gang recruiting.

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Mr. Johnson revealed the documents at a hearing on the dangers posed by MS-13.

Sen. Claire McCaskill, the ranking Democrat on the committee, said she wants to get rid of gangs, but criticized the release of the whistleblower documents, saying it may have hurt active investigations or otherwise dented efforts to get the gang members out of the country.

“I have concerns that these documents were released so quickly,” she said.

The UAC surge has long been suspected of providing a new pool of recruits for MS-13 and other gangs in the U.S., though the actual numbers are unclear.

There have been a series of anecdotal reports, however, of UAC who were placed into community schools and ended up taking part in violent gang activities.

“The gangs surf the internet, building dossiers on potential recruits,” Montgomery County Police Chief Thomas Manger said.

He said that while local gangs kill over drug disputes, MS-13 kills because of perceived gang ties or to enforce discipline within the gang.

“The homicides related to MS-13, it’s just because we can, and we will and because of the fear that instills,” Chief Manger said.

MS-13 has also begun extorting money from local immigrant families and businesses, warning of reprisals to their families back home if those living here in the U.S. don’t pay up, Chief Manger said.

MS-13 is notoriously violent, with beheadings or amputations by machete a not-infrequent punishment inflicted on those that defy the gang.

Scott Michael Conley, a police detective in Chelsea, Massachusetts, said the gang is well-established on the west coast, where it is heavily invested in drug trafficking, and in El Salvador, where its extortion network is deep.

Detective Conley said MS-13 is still developing on the East Coast, biding its time while trying to get its leadership in place.

“Once they establish that leadership base you’ll start to see a more sophisticated gang,” Detective Conley said.