The world knows ISIS fighters from their horrific acts of terrorism and brutal propaganda videos, in which masked men appear on a foreign landscape and execute their victims. But there is another side to the ISIS menace, hidden in the most familiar of places: American cities and suburbs, where recruits live among unsuspecting relatives and neighbors, communicating with the terror group through social media.
Watch "On Assignment" on NBC tonight at 7 p.m./6c for Richard Engel's exclusive report on the Americans who left the United States to join ISIS
American law enforcement officials estimate that roughly 250 Americans have tried to join ISIS. Most of them never left the United States, raising fears of more homegrown attacks like the one in December in San Bernardino, California. Assistant Attorney General John Carlin, the Justice Department's top national security officer, told NBC News that his agency has open investigations in all 50 states.
But a few dozen of those American recruits have made the trip to ISIS's heartland in Syria and Iraq.
In March, NBC News was given a thumb drive by a man claiming to be an ISIS defector. The drive contained the names and snippets of biographical information of more than 4,000 ISIS foreign fighters who entered Syria in 2013 and 2014. The documents, effectively ISIS personnel files, have been verified by West Point's Combating Terrorism Center and other counterterrorism specialists.
Through the documents, NBC News has identified at least 15 American citizens or residents who have joined ISIS overseas. They fit no particular pattern. Some are from poor Muslim immigrant families. Others had what can be described as privileged backgrounds. Three have Somali backgrounds. One was a Latino convert to Islam. They lived in small towns and cities in New York, Texas, California and places in between.
When he turned 18, Raihan moved in with Nasrin and Khan, and together they moved from Ohio in May 2014. Two months later, according to the ISIS documents, he entered Syria at Tel Abyad with Khan. Raihan, who took the battle name Abu Abduallah al-Amriki, was killed in Syria, according to family and a senior U.S. intelligence source.
It turned out that she'd moved to California to marry Khan, whom she'd met online, according to relatives. She became a devout Muslim, covering her face and staying away from other men, relatives said. The couple returned to Ohio, and she continued on to Ohio State. Then, in May 2014, she and her husband, along with her younger brother, left. In July, the two men entered Syria, according to the ISIS documents. Nasrin ended up working with Khan at a hospital in ISIS-controlled Raqqa, Khan's father told NBC News. Now 24, she is the mother of a 10-month-old girl, relatives said.
An acquaintance, fearing Khan was getting involved with jihadists, contacted the FBI, but what came of that tip is unclear. The couple also traveled to Kenya, prompting interest from the FBI, Nasrin's younger brother, Rasel Raihan, would later tell a friend.
Months after Raihan moved in with the couple in Ohio, the three of them left the U.S. for Syria. The last their families heard, Khan, 24, and his wife were working in a hospital in ISIS-held Raqqa and had an infant daughter.
For full details on how Khan, Nasrin and Raihan joined ISIS, check back at NBCNews.com at 6 p.m. and watch "On Assignment" at 7 p.m./6c
He was raised Catholic, but at some point, he stopped going to church and adopted Islam, relatives told NBC News. He began wearing robes on Fridays and attending services at the South Valley Islamic Center. The imam there said he was "very quiet" and never socialized. Then, two or three years ago, he stopped showing up.
His family told their landlord that he had joined the military. But the ISIS documents show that he entered Syria at Tel Abyad in March 2014, when he was 24. His whereabouts since then are unclear. One of his sisters told NBC News, "My brother was a good person."
Born in India and raised in Kuwait, Rahman, 22, came to the United States to study computers at Collin College in McKinney, Texas. School officials confirmed he was a student there from January 2012 to May 2014, and records show he had a perfect grade-point average for at least one semester.
His father told NBC News that he last heard from his son by phone in 2014 and later went to the Indian Embassy in Kuwait to report him missing. "I am searching for him for almost two years. I am really frustrated," he said. Rahman's ISIS file listed his fighting name as Abu Salman al-Hindi. He told his handlers he was highly educated in computers and was skilled in manufacturing, but didn't work because he "has a beard in America."
His father said he could not believe he had joined ISIS because he was so obsessed with his studies. "I don't believe he can have a mind to do anything else," he said.
When al-Haymar called his family in Morocco in March, they thought he was in Virginia, where he had been living since 2009 with his wife, son and daughter. The ISIS files, though, say that in March 2014, the 31-year-old entered Syria through Tel Abyad on the Turkish border; his form notes that he worked as a taxi driver and intended to be a fighter, not a suicide attacker. His mother and brother-in-law were shocked to learned his name was in the files; while he was an observant Muslim, they said that he never expressed any extreme beliefs.
Alliu's ISIS file says he was born in Albania, but his family moved to the United States in 2009, after a few years in Canada. He ended up in Boston, and worked as a cook before he arrived in Syria in May 2014, a couple months before his 20th birthday. His application said he'd been inspired by Albanians who'd come before him and that he asked to be part of a group called the Albanian Brigade.
NBC News was unable to reach Alliu's family through the contact number in his ISIS file, but according to court documents, an Albanian with the same name filed petitions for asylum in both Canada and the United States that said he fled Albania because he feared persecution for his political beliefs and for evading the draft. The Boston-based lawyer for the asylum applicant said she won a reprieve from deportation for him in July 2014 but when she tried to inform him, he had disappeared.
Hanad Abdullahi Mohallim
American law enforcement officials say that Mohallim recruited other fellow Somali-Americans in Minnesota, including some who werearrested a year ago for allegedly plotting to travel to Syria. Prosecutors have said that Mohallim sent his friends images on Facebook that showed him holding weapons. Mohallim was reported killed in an airstrike.
Nur left Minneapolis for Turkey in May 2014, according to prosecutors, who charged him afterwards with conspiring to provide material support for ISIS. In the two months before his departure, Nur had become "much more religious," urging that his family pray more and wear more traditional Muslim clothing, according to a criminal complaint. He also began to talk about jihad. After leaving, he communicated via Facebook with a friend back home, saying, "im not coming back," according to the complaint. He also allegedly tried to recruit other men back in Minneapolis to follow him, posing for pictures on Facebook holding weapons, prosecutors say.
Nur's fate is unknown; federal authorities have cited a recording of a March 2015 conversation among Minneapolis recruits in which one of them mentioned that he hadn't heard from Nur in a while and may have been killed.
Although the name is spelled differently in the file, the details match up with Jama, who has been identified by authorities as a Minnesota man who left home to join ISIS when he was 21. According to court papers, Jama saved up $5,000 for the trip; his first attempt was thwarted when the family of his traveling companion intervened. Instead, in June 2014, he took a Greyhound bus to New York City and boarded a flight to Turkey, the court papers say. He is believed to have been killed in August 2014.
McCain wrote that he wanted to be a fighter, and that's what he became, calling himself Abu Jihad the American. Five months after his arrival, he was killed in a battle with Syrian rebels. On his Twitter bio, he left this message: "It's Islam over everything."
Ahmad Hussam Al Din Fayeq Abdul Aziz
Aziz didn't say where in America he was from, but noted that he was single, had a high school education, and had worked as a furniture salesman before arriving in Syria at Tal Abyad in March 2014. He gave his fighting name as Abu Bakr Alsinawi and left contact numbers for his father and uncle, saying he hoped to become a suicide attacker. It is unclear whether he did. NBC News was unable to locate his family or verify his identity.
Muhammad told ISIS he'd lived in Brooklyn and was married. He said he arrived in Syria in 2014, when he was approximately 26. He'd entered the country at Jarabulus, on the Turkish border, helped by members of the Soldiers of al-Aqsa, a battalion of foreign fighters. NBC News was unable to locate his family or verify his identity.
Sari Abdellah al-Kambudi
In his ISIS application, al-Kambudi said he was married with six children, was a computer engineer, and had traveled extensively through the Middle East and Asia. He said he was Cambodian but lived in Washington, and the contact number he gave had a Seattle exchange. He said he'd made contact with ISIS through Twitter, and reached Syria in August 2014, when he was approximately 28. NBC News was unable to contact his family or verify his identity.
Tracy Connor and Jon Schuppe also contributed to this report.
This article originally appeared on NBCNews.com.