The El Salvadorian criminal enterprise MS-13 which saw two of its members sport sickening smiles in court for the 'satanic' kidnap and murder of two female teens in Houston is estimated to have between 10,000 and 20,000 members nationwide.

MS-13, also known as Mara Salvatrucha, is believed to have a presence in 46 states according to an independent study done by University of Pennsylvania.

The gang is thought to be behind a string of murders and violent crimes although the exact figures are not known.

Thirteen alleged members, ten of whom are illegal immigrants, were arrested in connection to seven slayings in Long Island over the last year.

The latest murders have begun to push into the public eye the decades of evil perpetuated by the group - and the terror of those living among it which sees female members inducted by being gang raped.

The first and only criminal organization in the United States to be named as 'transnational' by the FBI, MS-13 operates on the motto of 'Kill, Rape, Control,'

MS-13, also known as Mara Salvatrucha has between 10,000 and 20,000 members nationwide, and have a presence in 46 states

Miguel Alvarez-Flores (right) smiles and waves at news cameras in Houston court on Wednesday, as the gang leader known as 'Diabolico' and his partner 18-year-old Diego Hernandez-Rivera (left) faced charges of aggravated kidnapping and murder

The MS-13 is the first and only criminal organization in the United States to be named as 'transnational' by the FBI and operates on the motto of 'Kill, Rape, Control,' Massachusetts Attorney Carmen Ortiz said.

The group was started nearly 20 years ago in Los Angeles after millions of immigrants from El Salvador came to the United States after a violent civil war left over 100,000 dead.

The gang has since spread all over the country, and are known widely as the best killers - due to their exceedingly brutal weapon of choice, a machete.

The group was started nearly 20 years ago in Los Angeles after millions of immigrants from El Salvador came to the United States after a violent civil war left over 100,000 dead

They have since spread all over the country, and are known widely as the best killers - due to their exceedingly brutal weapon of choice, a machete

Weapons on display at the U.S. Attorney's office following the arrests of gang members in Boston, in 2016, when 56 members of the MS-13 gang were charged on murder-related crimes

WHO ARE THE MS-13? The ultra-violent street gang MS-13 was the first to be designated as an international criminal group. With as many as 20,000 members in 46 states, the gang has expanded far beyond its initial roots. Members are accused of major crimes including murder, kidnapping, prostitution, drug smuggling and human trafficking. Major roundups of MS-13 members have taken place across the country since the early 2000s. In Charlotte, N.C., 53 gang members were arrested as part of Operation Fed Up, which targeted MS-13 members. In 2016, 58 members of MS-13 were rounded up as part of 'Operation Mean Streets'. Previously, two of their most notable killings include stabbing to death a federal witness in 2004 and chopping the hands off of a 16-year-old boy in 2005. Advertisement

The two young women killed by MS-13 in Long Island, Nisa Mickens, 15, and Kayla Cuevas, 16, were beat with bats and hacked with machetes so severely that their bodies were unrecognizable. They were both students at Brentwood High School.

On Wednesday, gang leader Miguel Angel Alvarez-Flores, 22, who also goes by 'Diabolico,' and 18-year-old Diego Alexan Hernandez-Rivera appeared in Houston court where they laughed, smiled and waved to cameras while facing charges of aggravated kidnapping and murder.

Prosecutors said the two kidnapped a 14-year-old girl after she left school around February 2, holding her in an apartment where she was raped by a gang member.

Authorities say she was held against her will there for two weeks, given drugs and alcohol to keep her disoriented, and sexually assaulted by some of the men.

She also said she was held down while Flores tattooed a giant grim reaper from her knee to her foot, the Houston Chronicle reported.

Nisa Mickens, 15, (left) Kayla Cuevas, 16, (right) from Long Island, New York, were were ambushed by a carload of other teens on September 13 and killed. Thirteen members of the MS-13 gang were arrested on Thursday in connection with the slayings

Two known MS-13 gang members, formerly of El Salvador, Miguel Alvarez-Flores, left, and Diego Hernandez-Rivera appear in court in Houston on Thursday for the murder of one young woman and kidnapping of another

The gang operates by targeting children as young as eight in urban areas heavily populated with Central American immigrants.

They use a complex system of identifying at-risk youths, usually who have difficult home-lives, to target, exploit, and turn them into killers.

Those who become part of the gang must first be 'jumped in,' which includes a savage beating for 13 seconds by all members of the gang.

Sin Nombre, a film which documented the reality of MS-13 gang members, showed the induction of a 13-year-old boy named 'Smiley,' who was picked up in Boston in 2016 when 56 members of the gang were arrested

If the hopeful inductee survives, then they will be welcomed with 'open arms,' according to an independent study done by University of Pennsylvania researcher Albert De Amicis.

Brenda Paz was a criminal informant for the FBI. She gave first-hand insight to authorities that allowed a better understanding of MS-13's complex operations

To build respect from fellow gang members, new inductees are encouraged to execute 'chavalas' - who are enemies of MS-13 and often rival gang member.

Sin Nombre, a film which documented the reality of MS-13 gang members, showed the induction of a 13-year-old boy named 'Smiley,' who was picked up in Boston in 2016 when 56 members of the gang were arrested.

In a particularly graphic scene, another gang member holds Smiley's hand steady while he shoots a chavala in the head. The gang leader then fed the victim's entrails to his dog.

In the case of the two Brentwood high school girls, Robert L. Capers, the United States attorney for the Eastern District of New York, told the New York Times how Kayla Cuevas had been 'feuding' with gang members at school and on social media.

It's possible they could have viewed her as a chavala and an outlet for retribution - and an opportunity for a new member to prove their stripes.

Women can also be inducted into the gang by being jumped in or 'sexed in,' which generally carries a lower social status.

Being 'sexed in' entails being raped by every member in the gang until they are sexually satisfied.

Many girls who undergo this horrific torture end up prostituting themselves for the gang across the country, according to Brenda Paz, who was a criminal informant for the FBI. She gave first-hand insight to authorities in the early 2000's that allowed a better understanding of their complex operations.

After being exposed as a 'rat,' she was stabbed to death on the alleged orders of her ex-lover Dennis Rivera, who was an imprisoned former gang leader. She was 17 years old and four months pregnant. He was later acquitted for the crime.

After being exposed as a 'rat,' Brenda was stabbed to death on the alleged orders of her ex-lover Dennis Rivera, who was an imprisoned former gang leader. She was 17 years old and four months pregnant. Rivera was later acquitted for her murder. This knife was shown preceding a news conference in Boston after 58 MS-13 members were arrested

To build respect from fellow gang members, new inductees are encouraged to execute 'chavalas' - who are enemies of MS-13 and often rival gang member

MS-13 has also reportedly been contracted by El Chapo to aid in his drug war and low-level distribution. De Amicis' research detailed statements that MS-13 and other Central American gangs have attempted to expand their own drug routes through collaborations with Mexican gangs like Los Zetas and the Sinaloa Federation drug cartel.

In addition to their lethal presence in the United States, Mexico, and Canada, MS-13 is also firmly rooted in their home of El Salvador, Guatemala, Honduras, and Nicaragua.

Here in the US, Long Island, specifically Brentwood, seems to be receiving the brunt of MS-13's deadly force. Eleven gang killings in Suffolk County last year alone were attributed to MS-13.

Tensions have reached a breaking point between parents and school officials, as the recent deaths of Nisa Mickens and Kayla Cuevas have devastated the already gang-torn community.

In addition to their lethal presence in the United States, Mexico, and Canada, MS-13 is also firmly rooted in their home of El Salvador, Guatemala, Honduras, and Nicaragua

Evelyn Rodriguez, mother of Kayla Cuevas, 16, stops to talk members of the press gathered outside U.S. District Court Thursday, March 2

They worry that the increase of illegal immigrants from across the country's borders have indeed brought in a new wave of increasingly violent gang members.

'There's been a huge influx, to be honest with you,' said Ray Mayo, the president of the Brentwood Association of Concerned Citizens.

'It seems like a whole new set of gang members who have stirred the pot up.'

For El Salvadorian residents in Brentwood, it brings back old feelings of terror from the country's civil war that sparked their immigration two decades ago.

One woman who wished to be unnamed told the New York Times: 'It makes me feel bad that people think this of all Salvadorans,'

'Violence was the reason I left — when they killed my brother. And now we are experiencing the same violence.'