Irene Bamenga had her plane ticket to go home. The 29-year-old, carrying a bag of medications to treat a life-threatening heart condition, had planned to return to France and wait out her application for permanent residence in the United States before rejoining her husband in Lynn.

Bamenga had stayed in the United States much longer than she was supposed to under a visa waiver program, but she was exactly the kind of person immigration agents are officially encouraged not to put in jail: She had no criminal record, a husband in the country legally, and a heart condition — and she was trying to leave on her own anyway.

Still, when border agents discovered Bamenga was here illegally as the couple tried to drive to Canada for her flight in July 2011, the young woman ended up in handcuffs and a belly chain, joining 33,000 other immigrants across the country that day who were prisoners of the Department of Homeland Security.

Twelve days later, Irene Bamenga was dead.

“She was terrified,” her husband, Yodi Zikianda, recalled of the day of her arrest. “She was telling me, ‘I cannot stay in jail.’ ”

He repeatedly called the phone number that border agents gave him to try to help his wife get out or to at least get her medications, but he couldn’t get beyond taped messages.

If Bamenga had been accused of a crime, she would have been entitled to a public court hearing within hours of her arrest, giving her a chance to state her case and to plead for treatment of her congestive heart failure. But as a prisoner awaiting deportation inside the nation’s most secretive detention system, she had no right to a hearing or a court-appointed lawyer. Most important, she had no reliable access to the six medicines that she needed to stay alive.

For days, as Bamenga was transferred from one New York jail to another, she either got no medications or reduced doses, even though she told jailers she was struggling to breathe and had palpitations. She even filed a written request for medical help. Finally, on the 12th day of detention, her cellmates found her lying on her bunk, eyes wide open, not breathing — dead after receiving what the immigration system’s own investigators concluded was poor medical care.

“It was all wrong,” said Zikianda, a parking attendant in Boston and an engineering student at Wentworth Institute of Technology. “Why couldn’t they just let her go?”

Irene Bamenga paid the ultimate price for her encounter with immigration authorities, but countless others have suffered from its toxic combination of insensitivity and lack of public accountability, a yearlong Globe investigation has found.

Every day, Immigration and Customs Enforcement, or ICE, detains more than 10,000 immigrants who have no criminal records and sometimes deep ties to the United States, holding them for weeks and often months in jails where they have fewer rights than criminals and little access to the outside world. It is a system that separates parents from young children, locks away the elderly during their twilight years, and sometimes puts the sick at great risk.

Yet, that same agency has released 8,500 criminals — including as many as 201 murderers — to US streets over the past four years because their home countries wouldn’t take them back, as the Globe reported on Sunday.

Such inconsistencies fester in a fast-growing detention system that provides little information about the people it arrests. ICE’s network of detention centers — almost all of them originally designed to house criminals — has quadrupled in size since 1995, but ICE doesn’t release even the names of detainees, purportedly because it needs to protect immigrants’ privacy.

The information that ICE officials do release, such as the calculation that detainees typically remain locked up less than a month, masks painful individual stories. On one day last January, nearly 3,000 immigrants had been locked up for more than six months — and close to 900 for more than a year — while they went through the deportation process, according to ICE records released last month to the American Civil Liberties Union.

Despite the official secrecy, however, immigration detention stories do surface, making it clear why the acronym ICE strikes fear into so many immigrants’ hearts: