Trump and politicians tighten security as police hunt Charlie Kirk's killer

National Guard troops seen in New York on Thursday

The US political world has been on edge and concerns about security have been rising across the country since conservative activist Charlie Kirk was fatally shot at a university in Utah.

President Donald Trump's security teams changed the location of his speech to commemorate the 11 September 2001 attack on the US, and are tightening security for a baseball game in New York he plans to attend later on Thursday.

Several universities have been locked down after receiving anonymous threats, and some high-profile figures are boosting their protection, with at least one vowing to carry their own weapons.

Authorities are still searching for the person who shot Kirk, adding to the unease.

President Trump's remarks at the Pentagon, one of the scenes of the major attack 24 years ago, were moved from a more public space to an interior courtyard on Thursday morning "out of an abundance of caution," officials said.

The Secret Service is also exercising caution for the game at Yankee Stadium, and has warned that people in the area "can expect to see an increased law enforcement presence".

Those attending the game have been advised to arrive early and budget extra time to get through long queues that are expected to form due to what stadium officials described as "enhanced security measures".

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As the search for Kirk's killer continued, and people debated possible motivations behind the shooting of the conservative firebrand, both sides of the political spectrum grappled with the potential for violence.

In Washington DC, a bomb threat was made to the Democratic National Committee headquarters, trigging a police search of the building where a pipe bomb was discovered on 6 January 2021.

US Capitol Police have determined that it was not a credible threat.

Separately, New York Democratic Congresswoman Alexandria Ocasio-Cortez - a frequent target of Kirk's criticism - cancelled an event in North Carolina.

Her team cited security concerns and said the cancellation was "out of respect for Kirk".

Conservative political commentator Ben Shapiro also withdrew from a previously scheduled event outside Los Angeles.

Republican Congresswoman Nancy Mace, who is running for governor in South Carolina, told reporters that she would not hold any public events anytime soon due to security concerns.

She added that she planned to start carrying a firearm whenever legally-permitted.

"I'll start carrying again when I'm back home," she said. "In other states, I will have a firearm on my person all the time, and I will have security."

The US Naval Academy in Maryland was placed on lockdown on Thursday evening, with sailors from the US Navy being told to shelter in place as police searched for an unconfirmed threat.

"The base is on lockdown out of an abundance of caution," a Naval Academy spokesperson said. "This is a developing situation, and we will provide updates as they become available."

Several universities across the South with predominantly black student bodies were forced to lock down after receiving threatening messages.

Some of the historically black colleges and universities - known as HBCUs - have cancelled activities after lifting their lockdowns.

Southern University and A&M College in Baton Rouge, Louisiana, and Virginia State University in Chesterfield, Virginia, both had everyone on campus lock into buildings after receiving threats.

Alabama State University in Montgomery, Alabama, cancelled all activities for Thursday and posted on social media that people would be permitted to leave their buildings where they were sheltering once law enforcement cleared them.

Hampton University in Hampton, Virginia, "ceased all non-essential activity effective immediately" for Thursday and Friday due to a "potential threat". Its social media posts did not specify what the threat was, but encouraged remote meetings and for people on campus to "minimise their movement".

Other universities were also hit by hoax threats, which have become more prominent in recent years.

A large police response was called to the University of Massachusetts in Boston after a caller reported a shooting. Officials say they have not confirmed that any shooting took place and deemed the campus safe. The university said all classes and activities would be cancelled for Thursday.

US Democratic House Minority Leader Hakeem Jeffries said the threats were "yet another indication that the explosion of hateful extremism is out of control".

"These attempts to intimidate everyday Americans will not stand. We need leadership at this moment that brings the country together," he said.