The explosion at an Ariana Grande concert in northern England on Monday night that killed at least 19 and injured more than 50 "has the workings of jihadist terrorism," according to former U.S. law enforcement officials with experience in counter-terrorism.

Police said the explosion is being investigated as a terrorist attack, unless new information proves otherwise. Britain’s terrorist threat level has been set at “severe” in recent years, indicating an attack is likely.

There was no immediate confirmation of who was behind the carnage.

“It’s got the works of jihadist terrorism—you have an American performer in the U.K. and notably, extremists frequently target children because that hurts and weakens people, adults,” former FBI special agent and former FBI national spokesperson John Iannarelli told Fox News. “The location of the bombs are a tell-tale sign—the bombs were outside the secured areas and planted in the lobby areas.”

Iannarelli added: “At some point, you need to decide where your security begins and ends—it’s not possible to cover everything, and terrorists know this.”

Witnesses reported hearing two loud bangs coming from the main foyer where the guests enter with their tickets, which, reportedly, was outside the secure zone of the arena.

Former deputy assistant secretary at the Department of Homeland Security under President George W. Bush, James Norton, told Fox News that the arena was an “obvious soft target.”

“Soft targets are stadiums, airports, large gatherings—this shows the similar map that ISIS has followed,” Norton told Fox News. “This is the scary thing—the soft targets, the stadiums, have screening getting in, but there isn’t typically screening at the front end of the stadiums and the arenas –this could potentially be another attempt by them.”

Norton also noted the significance of the timing of this attack.

“The president traveling through the Middle East could be timed around this—his trip was well-broadcast in advance, and this could easily be a very well-planned attack,” Norton said.

Former member of the FBI National Joint Terrorism Task Force, Steve Rogers, agreed with Norton, and told Fox News that terrorist attacks are “well-planned.”

“We know that when terrorist attacks are planned, there is a certain reconnaissance—an individual looks for a time and place,” Rogers said. “This is almost certainly a terrorist attack--of course, we’re waiting for responsibility, but somebody within the next 48 hours will certainly claim responsibility.”

But deputy assistant to President Trump, Sebastian Gorka, said that the fatal blast in Manchester could be timed with the date of another fatal attack in the U.K. four years ago, underscoring the importance of “dates” to radical jihadists.

Gorka tweeted, “Manchester explosion happens on 4th anniversary of the public murder of Fusilier Lee Rigby. Dates matter to Jihadi terrorists.”

Rigby was the British Army soldier stabbed and hacked to death in London in 2013 by two Muslim converts. That attack also happened on May 22.