President Trump’s prime-time speech Tuesday night addressed what he has called a national and humanitarian crisis at the southern border.

Trump has demanded $5.7 billion to fund a border wall, part of a major campaign promise that he has been unable to muscle through Congress.

Border apprehensions remain at their lowest levels in decades, and federal statistics show that most who enter illegally do not have criminal records.

But the southwest border is facing a crisis that Trump did not foretell when he vowed to crack down on illegal immigration: An influx of Central American families fleeing poverty, gang violence and threats. Two young children died in federal custody last month, and border officials say they are not equipped to deal with the thousands of parents and children who continue to flow in.

Here are some basic questions and answers about border crossings:

Q: Has the number of people crossing changed?

A: The best way to measure illegal border crossings is by the number of people arrested by the U.S. Border Patrol after entering. In 2000, border agents arrested more than 1.6 million people along the U.S.-Mexico border. Agents arrested 303,916 in fiscal 2017, the lowest number since 1971. In fiscal 2018, which ended in September, the number rose to nearly 400,000.

Q: Who is crossing?

A: In the 1990s and 2000s, detained migrants were generally adult men from Mexico who could be easily deported after a short stay in holding facilities. Now, the majority of crossers are Central American families or unaccompanied minors who are difficult to quickly remove because of federal laws prohibiting the swift deportation of unaccompanied minors from countries other than Mexico and legal limits on how long children can be detained with their parents.

U.S. Customs and Border Protection detained 25,172 family members in November, a record high, and 5,283 unaccompanied minors. The groups accounted for nearly 60 percent of that month’s border arrests. Last fiscal year, agents arrested 107,212 family members — meaning at least one parent and one child — compared with 14,800 five years earlier.

Q: How many of those crossing are seeking asylum?

A: U.S. immigration courts received nearly 120,000 asylum claims in fiscal 2017, four times the number in 2014.

The Trump administration argues that migrants are filing frivolous claims, aware that they are likely to win release in the United States as their cases move through the backlogged immigration courts.

Advocates say migrants are fleeing some of the world’s most dangerous nations and have legitimate asylum claims. Some argue that the solution is to assign more judges to handle the cases.

Q: How many people are in detention facilities?

A: Five years ago, the government was detaining an average of 33,000 immigrants a day. As of Jan. 1, U.S. Immigration and Customs Enforcement held 48,019 in custody — about 8,000 more than Congress has authorized. Most are adults; 1,928 are detained families.

Officials say most detainees aren’t eligible for release due to criminal records or other reasons. Because immigration records are not available for public scrutiny, it’s impossible to independently confirm that.

Q: How has staffing at the border changed?

A: Trump promised to increase Border Patrol and ICE staffing substantially. But the number of border agents has been fairly flat, at around 20,000, double the number who were patrolling when arrests peaked in 2000.

Q: What’s the pace of cases in immigration courts?

A: The Trump administration set a goal of slashing the immigration court backlog in half by 2020. But the number of immigration judges has hovered at about 400. And despite new production quotas of 700 cases a year for each judge, the backlog has worsened.

The Justice Department, which runs the courts, estimated a backlog of 760,000 before the partial government shutdown that began in late December.

Trac, a Syracuse University organization that publishes court data, estimates that the backlog has surpassed 1 million.

Q: Is there currently a wall along the southern border?

A: There is a barrier along 650 miles of the 2,000-mile border. Some call it a fence. Others call it a wall. Border Patrol agents say the wall helps control foot and drug traffic and shields the United States from high crime in cities such as Juarez, Mexico.

Advocates for immigrants say the wall will not stop migrants, however, and instead is shunting illegal border-crossers to more barren, dangerous locations such as Antelope Wells, N.M., where 7-year-old Jakelin Caal was apprehended with her father in December. She died 27 hours later, officials said, of shock and dehydration. On Christmas Eve, Felipe Gomez Alonzo, 8 , died after several days in U.S. custody.

The Government Accountability Office has recommended that the Border Patrol develop metrics to determine how fencing contributes to border security.