Prison escapes are hard. How did Cavalcante and Kahlife do it?

According to experts, however, neither example is particularly representative of the vast majority of escapes, which tend to be spontaneous, attempted during transit or a temporary stay at a non-prison facility, and end in capture. One study, published in 2016 in the Prison Journal, found that more than 92 percent of 611 prison inmates who escaped were recaptured.

As of Thursday, both fugitives were still at large. Here’s how they escaped and what makes their methods unusual.

The ‘crab walk’ escape

Convicted murderer Danelo Cavalcante, 34, broke out of Chester County Prison in Pennsylvania last week after scaling a wall, pushing through razor wire, and running across the facility’s roof, acting prison Warden Howard Holland said in a news conference.

In surveillance footage released by the local district attorney’s office Wednesday, a man can be seen rapidly hoisting himself between two prison yard walls before vanishing out of frame. Holland described Cavalcante as having “crab-walked” out of prison.

“That’s very unusual,” said David Wilson, a professor of criminology and former prisoner governor in Britain, in a telephone interview Thursday, recalling few other examples of prisoners using the “crab-walk” method of delicately jimmying one’s body upward between two walls to escape.

One of them was the notorious Scottish safecracker Johnny Raminsky, who broke out of Scotland’s Peterhead prison on at least five occasions between 1934 and 1958. “On the first three occasions he climbed through windows or trapdoors in the roof and scaled the perimeter wall,” a government minister said after the final attempt. The longest Raminsky managed to evade capture was 10 days.

Another bizarre, physically-taxing escape was staged by Robert Dale Shepard, 34, who in 1994 braided 48 strands of mint dental floss into a cord, using it to ascend the 18-foot cinder block walls that surrounded the South Central Regional Jail in South Charleston, W.Va. He was captured after 41 days, The Washington Post reported at the time, describing him as a “real-life Spider-Man.”

Schools close as manhunt for murderer who escaped prison nears a week

In London, a former soldier escaped from the Victorian-era Wandsworth prison, where he was awaiting trial over suspected terror offenses. His Wednesday breakout sparked an ongoing nationwide manhunt, prompting officials to beef up security at British airports and warn the public not to approach him.

Daniel Abed Khalife, 21, who is accused of placing fake bombs on a military base, escaped wearing a prison kitchen uniform, Cmdr. Dominic Murphy, head of the London Metropolitan Police’s counterterrorism command, told reporters. Citing an unnamed government source, Sky News reported that Khalife is believed by authorities to have clung to the bottom of a van that was delivering food to the facility, holding onto straps on the vehicle’s underside while it drove off.

Prison escapes are rare in the United Kingdom, where only 10 inmates have broken out of prison in the past five years, according to the Justice Ministry.

In 1965, Wandsworth was also the site of one of Britain’s most notorious prison breaks — that of the convicted train robber Ronnie Biggs. Along with three others, Briggs staged a daytime escape after outside associates threw rope ladders over the prison wall, hoisting him up. Biggs jumped into an awaiting van and made it to France, where he had plastic surgery, before fleeing to Australia and then Brazil. He voluntarily returned to Britain in 2001 in the hope of lenient treatment. Instead, he was sent to prison for the remainder of his term.

“Those escapes where the escaper tends to be longer at large are the unusual ones,” said Wilson — who laid out three stages of a successful prison break. “First is the escape. Second is the support immediately in the community. Third is — long term — how is the escaped prisoner going to evade detection for a long enough period of time.”

The importance of outside help makes it very difficult for escaped inmates, whose breakouts Wilsons says are frequently opportunistic and not carefully planned, to avoid eventual capture. “People simply don’t have the community infrastructure which is going to support them once they’re out of the prison wall,” Wilson said.

Hours after Khalife’s escape in the British capital, a man arrested on a murder charge in Washington, D.C., escaped from custody while being treated at George Washington University Hospital.

Authorities did not provide any information about how Christopher Patrick Haynes, 30, escaped from the hospital, in the 900 block of 23rd Street NW, at about 3:30 p.m. Police said he was last seen “with black handcuffs hanging from his right wrist.”

According to Wilson, inmate escapes from non-prison facilities where they have been temporarily taken by authorities are among the most frequent escape methods. “Many prisoner escapes happen when a prisoner is taken to hospital or when a prisoner is taken to court,” he said. These are spaces “where security is not necessarily at the forefront of the culture of that institution.”

In 2015, a federal prisoner escaped Inova Fairfax Hospital in Virginia, where authorities said he was being treated for suicidal behavior. After slipping his shackles, Wossen Assaye, 42, overpowered a guard, took her gun, and used her as a human shield while he fled the medical facility in a gown. The suspected robber was captured nine hours later in the District, after allegedly carjacking two vehicles.

Sarah Dadouch, Ben Brasch, Peter Hermann and Anumita Kaur contributed reporting.