In a decade-long killing spree, he may have killed as many as 350 girls.

But 20 years ago, Pedro Alonso Lopez walked out of a psychiatric facility in Colombia, never to be seen again.

Lopez was given the name "Monster of the Andes" after he started a killing spree which lasted from 1969 to 1980.

Almost all of his victims were girls aged between nine and 12, who he killed with impunity in Colombia, Peru and Ecuador.

In one three-year period, he was averaging three victims a week, luring them away from markets with trinkets, then raping them, strangling them and dumping their bodies in shallow graves.

Pedro Lopez killed potentially 350 girls but was released from prison nevertheless. (Supplied)

"I walked among the markets searching for a girl with a certain look on her face. A look of innocence and beauty," he told journalist Ron Laytner in 1994.

"She would be a good girl, always working with her mother. I followed them, sometimes for two or three days, waiting for the moment when she was left alone."

Lopez shared a common trait with many serial killers, a horrific childhood, according to the University of New Mexico's Dirk Gibson.

Pedro Lopez would lure girls away from markets in Colombia, Ecuador and Peru, then rape and kill them. (AP)

Dr Gibson wrote about Lopez in his book Serial Killers Around the World: The Global Dimensions of Serial Murder .

"At the age of eight he was turned out of the home after his mother caught him fondling his sister," Dr Gibson told nine.com.au.

"He was taken in by a man who turned out to be a pedophile, and he was abused for quite a while.

"Then an American couple tried to help him, but in a couple of years he was expelled from a school, then finally he was imprisoned, where he was gang-raped by a gang of four miscreants.

"Lopez claimed his murders were revenge for the gang rape he suffered in prison, and because of the mistreatment from his mother."

Pedro Lopez is considered the second worst serial killer of all time. (AP)

Lopez carried on his awful spree without drawing the attention of police.

"It was believed those children were human trafficked, stolen or sold on the market or that they ran away," Dr Gibson said.

"Often local law enforcement suspected serial killers last."

It took a natural disaster to literally uncover Lopez's crimes.

The discovery

A 1979 flash flood in the city of Ambato, Ecuador, unearthed the bodies of four young girls.

The sickening discovery put the city on alert, and locals spotted Lopez three days later leading a 10-year-old girl away.

He was arrested, where he confessed to the murders to an undercover detective posing as his cellmate.

(Nine)

He then agreed to take police to a mass burial site, where they found the graves of 53 of his victims. He would plead guilty to dozens more two months later.

But save for the bodies actually found, Lopez's claimed death toll should be taken with a grain of salt, said Dr Gibson.

"Many if not most exaggerate their victim count," he said.

"These aren't exactly honest people, and they are interested in glory, so they inflate their crimes.

"But at the very least, he killed up to 70 people."

The sentence

Lopez would be sentenced to the maximum prison term allowed under Ecuadorian law – sixteen years.

As soft as his sentence was, Lopez would incredibly be released two years early for good behaviour, quietly deported to his home country of Colombia in 1994.

Pedro Lopez spent just 14 years behind bars, in the now shuttered Garcia Moreno prison in Ecuador. (AAP)

In the end, he served just one month for each of the 110 victims he was found guilty of killing, and no time at all for the 240 girls he claimed to have murdered in Peru and Colombia.

Lopez long feared facing a firing squad in Colombia, but prosecutors were unable to make a case.

Instead he was declared insane and placed in the psychiatric wing of a hospital in the capital of Bogota in 1995.

Set free

Three years later, the hospital declared him sane and released him with a $70 bail bond.

Lopez absconded almost immediately.

Since then Lopez, officially the second worst serial killer of all time , has been on the run.

Luis Garavito, the worst serial killer of all time, has many parallels to Pedro Lopez. (Supplied)

The worst is Luis Garavito, another Colombian child rapist who killed at least 138 boys in the 1990s.

A warrant was issued in 2002 for Lopez's arrest in connection with a new murder, but he remains at large.

"In general, a serial killer will continue to kill until he's stopped," Dr Gibson said.