The U.S. Immigration and Customs Enforcement agency detains roughly 400,000 immigrants a year, according to the Department of Homeland Security -- roughly 40,000 people on any given day. Those who challenge their deportation typically stay locked up for months, even years, awaiting a court decision on whether they may stay in the United States.

The named plaintiff in the class-action case, Alejandro Rodriguez, spent three years in ICE detention without a hearing for his release.

According to the ACLU of Southern California, there are roughly 8,000 detainees on any given day who, like Rodriguez, have been detained for longer than six months.

The group includes legal permanent residents who are subject to deportation because they’ve been convicted of a range of crimes, and asylum seekers who have met the government’s initial criteria for protection from persecution. Both groups are subject to mandatory detention while they fight deportation in immigration court, but an immigration judge can order ICE to release detainees who pose little risk to public safety and have strong ties to their communities that make them a low flight risk. Typically, the judge will release the detainee on bond or on their own recognizance.

In 2014, a U.S. district judge in the Central District of California ruled in favor of the ACLU and required the government to hold a release hearing every six months, even for detainees who had been subject to mandatory detention. That decision was upheld in 2015 by the U.S. Court of Appeals for the 9th Circuit.

The Obama administration appealed to the U.S. Supreme Court, which heard oral arguments on the case in November 2016.

Nine Western states and two Pacific Islands in the 9th Circuit's jurisdiction are currently required to provide bond hearings every six months. Most other states have no specific timetable for holding release proceedings for immigrant detainees.

“We don’t have a decision today,” said Cornell immigration law professor Yale-Loehr. “Unfortunately, that means immigrants will continue to remain in detention until the Supreme Court's ultimate decision.”

A national standard setting limits on immigration detention could significantly thwart the Trump administration’s plan to end bond releases in all but the most extreme cases.

While the Rodriguez ruling merely established a right to a hearing every six months -- not a guaranteed release -- it shifted the burden of proof from the detainee to the government: Now immigration officials must provide evidence that the detainee is a threat to public safety, or likely to flee, in order to keep a person locked up.

The secretary of Homeland Security sought to end bond releases for immigrants subject to mandatory detention as part of stepped-up enforcement.

In a recent memo, Secretary John Kelly ordered ICE agents to keep people facing deportation locked up until they receive a final court decision on whether they are allowed to stay.

Kelly also asked Congress for $7.5 billion in discretionary funding for ICE in fiscal year 2018, in part to expand the number of detention beds for immigrants to 51,000.

If the justices ultimately strike down the decision, immigrants detained in California and the other 9th Circuit states could lose what advocates consider a key constitutional right -- protection from prolonged or arbitrary detention.