President Trump on Tuesday blamed former President Obama's immigration policies for the formation of MS-13, a gang the FBI has been closely tracking since at least 2004.

"The weak illegal immigration policies of the Obama Admin. allowed bad MS 13 gangs to form in cities across U.S. We are removing them fast!" Trump tweeted Tuesday morning.

The weak illegal immigration policies of the Obama Admin. allowed bad MS 13 gangs to form in cities across U.S. We are removing them fast! — Donald J. Trump (@realDonaldTrump) April 18, 2017

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Trump often slams Obama for his immigration policies. Earlier this month, the president tweeted that "immigration is plummeting" under his administration.

In the early 2000s, the MS-13 gang, created by Salvadorans, was recognized by the FBI as an increasing threat. The FBI in 2004 formed the MS-13 National Gang Task Force to closely track the gang.

The gang was in at least 42 states by 2008, according to the FBI. Obama took office in 2009.

During an interview that aired Tuesday on "Fox & Friends," Trump said illegal immigration is a "serious problem."

"We never did anything about it, and now we're doing something about it," he said.