U. S. official data suggests that as the US prison population has increased over the past few decades, prisons have also become far better at keeping the prisoners inside the walls and avoid escaping.

Data has showed that escapes from maximum-security prisons, such as the upstate New York prison where two murderers escaped last week, are exceedingly rare, statistics show.

According to the state officials, in the period of past five years i. e. between 2009 and 2013, just one inmate escaped from a maximum-security prison.

Officials dealing with prison security and administration said they face far more troubles like inmate suicides and violence directed at prison staff.

In 2013 itself New York prisons reported 161 suicide attempts and 645 assaults on staff members.

Martin Horn, a professor of corrections at John Jay College of Criminal Justice at City University of New York, said, "It's very rare - this happens very infrequently, which is why it's such a story. There weren't even enough incidents to study".

Across the nation, an estimated .6 million people were in state or federal prisons in 2013, the most recent year for which data is available.

On the other hand, an estimated 2,001 inmates were reported as 'absent without leave /escape' in 2013 by the federal Bureau of Justice Statistics.

In the New York escape last weekend, the two suspects evidently used power tools to drill precision holes through cell walls and pipes, said officials.

Kevin Tamez, managing partner for the MPM Group, a consulting firm near Philadelphia that does corrections work, said lapses could become more common as the USA's prison system ages.