Officials Warn Americans to Be on 'High Alert' Over Holiday Weekend Amid Melbourne Car Attack

About 14 people were injured as the driver, who is believed to have mental health issues, plowed into a crowd of holiday shoppers.

The Department of Homeland Security is cautioning Americans to be on high alert this holiday season as the world is shaken by yet another attack.

This time, authorities say, a lone wolf with mental health issues drove an SUV into a crowd of holiday shoppers, injuring 19 in Melbourne, Australia Thursday. Authorities say that the attacker did not have links to terror organizations. 

The attack comes just 10 days after a failed terror attack in a Manhattan subway tunnel.

Now, Homeland Security is about to release a new video, warning Americans to be extra vigilant.

"As you travel, shop and gather to celebrate the holiday season, we ask you to stay alert," the directive reads. "You play a critical role in keeping our nation and our neighborhoods safe."

Security expert Nick Casale joined Inside Edition in New York's Columbus Circle, which is teeming with visitors at this time of year.

"This is a new world we are into and you have to be extra sensitive and conscious of your environment," he told Inside Edition. 

ISIS has threatened New York directly with an ominous propaganda poster showing Santa Claus with a box of dynamite and the caption: "We meet at Christmas in New York soon."

Another ISIS poster shows Washington's National Cathedral in flames.

"You try your best to protect yourself," Casale added. 

He says to be alert and be conscious of your surroundings, adding that if you hear screeching tires or people screaming, take a look to see what people are reacting to and respond immediately to make yourself safe.