Attorney General Jeff Sessions on Friday ordered federal prosecutors along the southern border to adopt a "zero tolerance" policy when it comes to criminally charging all immigrants caught entering the country illegally, capping a week in which the administration railed against policies it said was drawing migrants here.

The new memorandum would swell already overburdened federal dockets at the border and expands a controversial program known as Operation Streamline that started in Del Rio more than a decade ago. Though simply being here without authorization is a civil offense, improper entry is a federal misdemeanor punishable by up to six months in prison, and the conviction can result in a felony if migrants return and potentially prevent them from gaining asylum.

Criminally prosecuting migrants allows the government to hold them in federal prisons, where there is more capacity than immigrant detention centers, before deporting them after they have served their sentence. It also enables the administration to separate families, as children cannot be held in prison, necessitating their placement in foster care and allowing the government to detain parents until they are deported.

"The recent increase in aliens illegally crossing our Southwest Border requires an updated approach," Sessions said in the memo.

The announcement came hours before President Donald Trump signed a memo ordering the end of so-called "catch and release," in which immigrants are freed to wait out their cases in the backlogged civil immigration courts.

He ordered government agencies to ensure that apprehended immigrants are imprisoned, including reporting steps taken to allocate money to build detention facilities near the border and listing military sites that could be used to hold immigrants.

Read more: Trump moves to end 'catch and release', prosecuting parents and removing children who cross border

Sessions' order, however, would likely have far greater consequence as the government under Trump already detains most immigrants it apprehends at the border.

But his administration, like President Barack Obama's before him, has struggled with how to handle the increasing number of Central American families and children. They make up the fastest-growing demographic of migrants coming here, though are among the most difficult to quickly deport.

A bipartisan 2008 law aimed at curbing human trafficking and a 20-year-old legal settlement protecting children in detention requires the release of most such families with children if asylum officers find that they have a credible fear of returning home.

If families are separated, however, parents can be detained until they are removed, fulfilling Trump's campaign-era promise of ending "catch and release."

Supporters say such a policy encourages migrants to come here, alleging that most of their claims to asylum are fraudulent.

Advocates warn separating families poses great risks as parents and children are not always reunited.

Godofredo A. Vasquez/Houston Chronicle

The speed of the criminal prosecutions and subsequent deportations at the border means advocates for migrant children in federal foster care are often unable to find parents prior to their removal. By then, advocates say, the children also have their own immigration cases and cannot simply be returned without court procedures, particularly if caseworkers argue it is against their best interest.

"What we are looking at is a separation of days and weeks and months, and some cases even years, because of the length of time a child's case can take," said Jennifer Nagda, policy director for the Young Center for Immigrant Children's Rights, which provides guardians ad litem.

The developments Friday came after a week in which Trump tweeted almost daily, criticizing what he has called "legal loopholes" that prevent the government from quickly deporting many immigrants. He also announced he was sending some 4,000 National Guard troops to the border, though they cannot by law detain immigrants.

"As ridiculous as it sounds, the laws of our country do not easily allow us to send those crossing our Southern Border back where they came from," Trump said this week. "A whole big wasted procedure must take place."

Read more: Trump administration tries to curb asylum

Bridget Cambria, a Pennsylvania immigration attorney who represents parents separated from their children, said the procedures were set in place by Congress to protect both potential asylum seekers and children.

Under the zero-tolerance policy, she said on Twitter, "children will be taken from their parents. Mothers will lose babies. Fathers will lose their children. They will enter our foster system. Watch for this disaster to play out, and for the lawsuits to pile up."

The American Civil Liberties Union filed a class-action lawsuit last month accusing the administration of illegally separating hundreds of immigrant parents and children who are seeking asylum.

Separately, the federal public defender's office of the Western District of Texas also contends that such prosecutions violate the due process rights of immigrant parents. Arguing on behalf of five Central American parents who were prosecuted in El Paso and had their children removed as a result, the office holds they pleaded guilty to the crime of illegal entry so that they can reunite more quickly with their kids.

Read more: ACLU challenges Trump administration practice of separating migrant parents and children

One of the parents, Blanca Vasquez, sought asylum at the El Paso border last fall after her husband, a Salvadoran military sergeant, was gunned down and gangs persecuted her family. Though she asked for refugee protection and requested a lawyer, the government removed her 13-year-old son and prosecuted the mother.

After four months in detention, she was released in February and allowed to reunite with her son after a judge agreed she had a potential claim to asylum. All of the other parents, however, were deported and most of their children remain with relatives or in foster care in the United States.

The administration in the past two months has ramped up its criticism of the 2008 law and 1997 legal settlement, known as Flores, suggesting increasing urgency as the number of families and children coming here has risen once more.

The Department of Homeland Security this week released statistics showing that more than 13,000 arrived in March.

"The crisis at our Southwest border is real," its statement said.

About as many arrived during March 2014 in what was seen as a surge of Central American families crossing the southern border, spurring a federal emergency as Obama's government struggled to house them all.

After Trump took office in January 2017, announcing a series of harsh immigration policies to deter migrants from coming, the number of such families dropped to record lows, about 1,200 in March of that year.

But they quickly began rising again and in recent months have almost reached Obama-era levels, even as the overall number of illegal crossings at the border are historically low, with about 415,200 apprehensions last year.

A Department of Homeland Security report last fall found that it is harder than ever to cross the border illegally, estimating that as much as 85 percent of attempted illegal border crossings are unsuccessful.

Johnny Hanson/Houston Chronicle

Advocates say the increase in families reflects the deteriorating security in El Salvador and Honduras, where gang violence, including that of MS-13, has made the countries two of the most dangerous in the world, after Syria and Venezuela, according to the global violence monitoring group Small Arms Survey. Political instability in Honduras, where President Juan Orlando Hernández took office in November after a highly contested election, is also drawing people here.

"The rising number of asylum-seekers requesting protection in Mexico or the United States is a humanitarian problem, not a security threat," wrote Adam Isacson, director for defense oversight at the Washington Office on Latin America, an advocacy organization.

Read more: Her husband murdered, her son taken away, a mother seeking asylum tells a judge, 'I have lost everything'

Sessions' order Friday tells prosecutors "to the extent practicable" to have "zero tolerance" when it comes to pursuing first-time offenders for improper entry. The order supersedes his previous instructions from last summer, which told prosecutors to prioritize such crimes. By the next month, May, immigration cases in federal courts had spiked 27 percent, according to federal data.

The attorney general in his directive referred to a zero tolerance pilot program in Texas that began under President George W. Bush, later expanding to six of the nine Border Patrol sectors on the southern border.

The sheer number of offenders charged with the misdemeanor means federal judges often have to see some 80 at a time, which critics have likened to a "cattle call in the courts."

Since there is no real defense to the crime, most plead guilty in a fast-tracked mass hearing that lasts about an hour and in which one taxpayer-funded lawyer represents the group.

While the legality of such expedited hearings -- a months-long federal process condensed into one -- has been upheld by appellate courts, advocates say the proceedings deprive immigrants of due process.

Read more: STREAMLINED: Trump pressing for mass criminalization of illegal border crossers

In particular, they say the zero tolerance strategy punishes asylum seekers in a violation of international treaties and can deny them an opportunity to make their claims for refugee status at all. The Department of Homeland Security's own Office of Inspector General identified this as a problem in 2015.

"Border Patrol does not have guidance on using Streamline for aliens who express fear of persecution or return," it said. "Its use of Streamline with such aliens is inconsistent and may violate U.S. treaty obligations."

Supporters of expanding immigration prosecutions say the prospect of prison time has contributed to historically low apprehensions at the border and deters migrants from trying to return. Border Patrol officials point to the agency's own figures showing that the number of people attempting to cross the border again within a year fell from 29 percent in 2007 to 14 percent in 2014.

The Government Accountability Office, however, has found problems with the agency's methodology, suggesting return rates basically remained unchanged when measuring over a more realistic time frame of three years and excluding immigrants who stayed in the United States in that period.

Marie D. De Jesus/Houston Chronicle

By 2013, U.S. Attorneys and federal judges along the border had largely moved away from such zero-tolerance prosecutions, frustrated by the time and expense they necessitated.

U.S. District Judge Sam Sparks, who oversees Austin in the Western District, has called the expenses involved in prosecuting all immigrants for illegal entry "simply mind-boggling," saying the taxpayer cost is "neither meritorious nor reasonable."

Detaining migrants on such charges alone costs some $1 billion a year, according to estimates by Grassroots Leadership, an advocacy group in Austin opposing incarceration.

By far the larger cost is the time, and resources, such a strategy expends.

The crime of illegal immigration now makes up almost half of all federal cases and 80 percent of the dockets in the Western and Southern Districts of Texas, which includes Houston, according to an analysis of federal data by the Transactional Records Access Clearinghouse, a data research group at Syracuse University.

Meanwhile, the second-most pursued offense by U.S. attorneys - going after drug-related offenses during the height of the nation's opioid epidemic - made up just 14 percent of all new federal cases last spring, according to Syracuse, falling to its lowest level in a quarter of a century.