The FBI has active terror investigations in all 50 states

Department of Homeland Security Secretary John Kelly warned Tuesday that the terror threat to the "as threatening" as it was on 9/11.

The expected military defeat of Islamic State in Iraq and Syria will decentralize the threat from the terrorist group and put the greater risk, as militants with citizenship in Europe return home and plot further attacks, Kelly said in a speech Tuesday in Washington.

"These numbers are lower because we've shown we are serious about border security, and enforcing our immigration laws", the DHS chief added.

"We don't know their intentions", Kelly said during an address at George Washington University.

Kelly said stepped-up enforcement has had a dramatic effect. It was the smallest number of arrests in a month in at least 17 years. If Congress doesn't act, a partial government shutdown would begin on April 29. And then rehabilitation. And then law enforcement.

In an interview on NBC's "Meet the Press," Kelly told host Chuck Todd that marijuana was "not a factor" in the war on drugs and argued that "arresting a lot of users" wasn't the right solution to the country's drug problems.

Obama shielded 750,000 young undocumented immigrants from deportation with a 2012 executive order.

"Let me be clear about marijuana", he said.

Kelly also defended immigration officials in the crackdown on illegal immigrants. "There are people who came here as children".

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Over the past few months, Sessions has hinted at the coming of a federal crackdown against the legal marijuana industry because he believes legal pot is contributing to a surge of violent activity as well as introducing more people to the risky world of opioid abuse.

Kelly said one of the biggest challenges is people who overstay their visas.

Kelly referred to the "vast tonnages" of marijuana and hard drugs that "TCOs" - transnational criminal organizations - move across the border from Mexico.

"They want to get here, they want to do us harm", he said. "Typically, what feeds the violence is the internet", Kelly said.

Kelly said DHS employees had been prevented from doing their jobs for "nearly a decade, disabled by pointless bureaucracy and political meddling, and suffered disrespect and contempt by public officials who have no idea what it means to serve". He also noted that "there's about 600 miles of barrier already in place". The backgrounds of potential DHS agents is "vetted extensively", he said.

Critics say the agency is too heavy-handed in enforcement operations, including arresting immigrants in the USA illegally who do not have criminal records.

There isn't a single state that the FBI doesn't have open terror investigations in, he said, adding that since 2013 there have been 37 plots to attack the US linked to ISIS.

"It was created to do what it was dropped on", he said, pointing to the bunker and tunnel complex targeted in a remote part of the country.