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US President Donald Trump has accused former president Barack Obama of being responsible for the formation of the deadly Mara Salvatrucha (MS13) gang in the US.

The gang is allegedly responsible for a string of murders, most recently accused of killing several teenagers in Long Island, although the gang's US activity pales in comparison to its prevalence in El Salvador, where it originated.

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In a tweet, Trump suggested the Obama administration was responsible for the gang's US presence, writing: "The weak illegal immigration policies of the Obama Admin. allowed bad MS 13 gangs to form in cities across US We are removing them fast!"

However, issues with MS13 in the US were identified by authorities prior to the time Obama assumed office in 2009 – with the FBI stating in the early 2000s that the gang was becoming a problem.

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Trump did not qualify exactly how he believed Obama had contributed to the growth of the gang, but has made spoken out about the need to deport people living in the US who are undocumented and have committed crime.

Attorney General Jeff Sessions said of the gang, speaking after the killings of the Long Island teenagers, and Trump's policies: "Transnational criminal organizations like MS-13 represent one of the gravest threats to American safety.

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"These organisations enrich themselves by pedaling poison in our communities, trafficking children for sexual exploitation, and inflicting horrific violence in the communities where they operate," he added in comments carried by Politico.

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