Hundreds of migrants nabbed by the border patrol after illegally crossing the US-Mexico border through Texas have been flown to Arizona and left at Greyhound Bus stations in Tucson and Phoenix during the past month.

The practice has drawn criticism from activists on both sides of the illegal immigration debate. Critics charge that released border-crossers will vanish into the woodwork. Immigrant advocates accuse the federal government of releasing migrants without providing enough basic necessities such as food and water on days that hover around 100 degrees F.

Arizona Gov. Jan Brewer (R) calls it "another disturbing example of a deliberate failure to enforce border security policies and repair a broken immigration system" in a letter to President Obama.

More deeply, however, it points to shifts in illegal immigration that could have major consequences for border security efforts.

The underlying problem is that Texas is replacing Arizona as the busiest sector for illegal border crossings. In fiscal 2013, agents in the Rio Grande sector caught 154,453 migrants, up from 97,762 the previous year. In Arizona's Tucson sector, which long felt the brunt of illegal immigration on the US-Mexico border, agents recorded 125,942 arrests last year, says Andy Adame, a Border Patrol spokesman.

Complicating matters further, many of the migrants coming to Texas are comparatively hard to deport. They are from Central America and are coming as families. Various studies conclude that crime, gangs, and poverty are driving people from countries south of the border. Those who make it to US soil agree, and add another reason: back home they've heard the United States may be lenient with illegal border-crossers who travel with children.

Ema Morales, dropped off at the Tucson bus station Thursday night with about 25 Central Americans, says that in her Guatemala town, "they're saying that women and children are allowed to stay."

The border patrol detained Morales and her two toddlers shortly after they crossed into Arizona. After a couple of days, Immigration and Customs Enforcement (ICE) drove her to the bus station with a group. Most were Guatemalan women and children of all ages.

Smugglers, who charge Central Americans $2,000 to $3,000 to get them across the border, also spread word among those from Guatemala, Honduras, and El Salvador of an existing opportunity for women to go free and to reunite with family members in the US, says Laurie Melrood, a family advocate working with migrants in Tucson. "But they don't tell them that they're in deportation proceedings as soon as they're captured by the border patrol."

Children have increasingly been coming alone, too. From October to May, 33,470 of the 47,017 unaccompanied minors caught along the entire Southwest border crossed through the Rio Grande Valley. In fiscal 2013, the total number of unaccompanied minors detained along entire border was 24,493, according to federal records.

On Monday, Mr. Obama described the influx as an "urgent humanitarian situation" that called for a swift response. Federal agencies are moving to provide temporary housing and other services.

These factors create problems for ICE, says spokeswoman Lori Haley.

"These families have minor children with them, and ICE has only one family detention center in Pennsylvania but otherwise we don't have detention facilities that can accommodate children," she adds.

Texas, caught unprepared by the shift, has not been able to cope.

"Because of the recent surge of Central Americans, unaccompanied juveniles, and family groups in south Texas, the border patrol is running out of processing space," says Mr. Adame.

That's where Arizona comes in. As the focal point for illegal immigration during recent years "the government expended a lot of resources to this area to include detention facilities or processing facilities," adds Adame.

After creating biographical files on the migrants and setting them up for deportation hearings, the border patrol in Tucson turns people over to ICE, which drops them off at the bus stations.

Immigration authorities for months have transported small numbers of undocumented immigrants at Tucson and Phoenix bus stations. But the larger crowds have turned a new page in Arizona's tumultuous history of illegal immigration.

The largest number of Texas detainees, about 400, landed here over the Memorial Day weekend and more arrived before and after, according to Adame. The border patrol's Rio Grande Valley sector is expanding processing space to accommodate the migrants, and it is unclear whether the flights to Arizona will continue.

Immigration authorities won't say whether Arizona should expect more arrivals from Texas, but in a Friday statement, Gov. Brewer said some 1,100 unaccompanied children will be transported here over the weekend.

"This is in addition to, not inclusive of, adults and family units, for which numbers have not been provided or any information given," she added.

Volunteers in Phoenix and Tucson cities have offered assistance, including food and shelter, to migrants stranded in unfamiliar cities while waiting for relatives living in the US to send bus fare.

Confusion has run rampant among migrants who are released without their belongings and have no idea where they are, says Dan Wilson, who volunteers with Casa Mariposa, a Tucson organization that lends border-crossers a helping hand. "A lot of them are traveling with small children and they don't have any diapers for the trip."

Governor Brewer expressed similar concerns to Obama: "I remind you that the daytime temperatures in Arizona during this time of year are regularly more than 100 degrees."

On the other hand, Mr. Wilson commends ICE for keeping migrants out of detention centers and for allowing them to live in communities where families can stay together.

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But Bob Dain, a spokesman for the American Federation for Immigration Reform, which promotes tighter immigration controls, blasted the Obama administration for its handling of the latest wave of illegal border crossings.

"Things have gotten out of hand, Congress needs to step up to the plate and compel this president to start enforcing the law," he says. "Otherwise you've got more and more surges of people coming in, overwhelming the system, and we've got a bureaucratic inability to process them."