Nearly two months since the lifting of a public health order that allowed the United States to swiftly expel migrants at the southern border, the number of migrants crossing into the country has not only sharply declined, but has also remained relatively low.

Since May 12, the average number of daily illegal crossings has been around 3,360, according to Department of Homeland Security data. In March 2022, that average was about 7,100.

The dip in crossings has been a welcome development for the Biden administration, which has experienced record levels of illegal migration during most of the president’s time in office.

Officials expected the expiration of the public health rule, known as Title 42, to bring an even higher number of illegal crossings, because they believed the change in policy could cause chaos if migrants who had not been able to seek asylum suddenly could. Those predictions, however, were made before the Biden administration introduced policies devised to blunt a potential spike. The increase in illegal crossings came in the days before the rule expired.

But officials say this lull, after nearly two years of higher-than-usual crossings, is not going to last. Determining the factors for increases and decreases in migration is not an exact science. Global migration trends, legal challenges and political changes in the United States and in countries most migrants emigrate from could all have an effect on which way the numbers will go. But here are some informed theories from government officials and outside experts based on the current conditions.

Officials believe that migrants have been in a wait-and-see mode since May 12, after the public health rule — which had been in effect for three years — was lifted and policies that restrict access to asylum and create new legal pathways were rolled out.

The new policies are already facing legal challenges, creating the possibility that a judge’s ruling could change one of them, pause it temporarily or end it completely. So many migrants are waiting to see whether the policies are here to stay.

They are also watching how others are faring at the U.S. border and whether they are encountering new obstacles in their quest to cross into the United States, said Falko Ernst, a senior Mexico analyst for the International Crisis Group.

“You might have people standing by because they’re hearing stories and they’re frightened” about the new policies making it harder to cross the border, Mr. Ernst said.

Officials believe fewer migrants are crossing illegally because they are taking advantage of a more structured and safer option to ask for a chance to seek asylum, as well as new legal pathways that the Biden administration has created for certain nationalities to enter the country.

In central and northern Mexico, migrants can gain access to a government app on smartphones, where they can apply for an appointment at an official port of entry at the U.S. border. While there have been some technical problems with the app, nearly 30,000 used it to make appointments in May alone, according to recent government data.

In addition, migrants from Cuba, Haiti, Nicaragua and Venezuela can apply for the chance to live and work in the United States for two years under a special humanitarian parole. In April, the Biden administration announced that migrants from El Salvador, Guatemala and Honduras would be eligible for a family reunification program. These programs, expected to start this month, allow certain immigrants seeking to reunite with immediate family members to enter the United States and later apply for a green card.

The measures Mexico has taken include limiting migrants’ abilities to travel throughout the country, making it harder for them to reach the U.S. border. Mexico is also flying migrants whom the United States has recently deported to southern parts of the country. This puts more distance between them and the American border, which makes it harder for the migrants who want to try to cross illegally again.

There is still extreme poverty, violence and political instability in the countries people are fleeing, including Haiti, Cuba, Venezuela, and Nicaragua and other Central American nations.

“I am confident there are a lot of people moving into the hemisphere, mostly headed this way,” Benjamine Huffman, a senior Customs and Border Protection official, said at a congressional hearing on June 6. “We see the news reports. We look at shelters that have people.”

As of June 14, there were about 104,000 migrants in northern Mexico, about eight hours from the U.S. border, according to an intelligence estimate the Biden administration gave in a recent court filing. And there are more along the route from Colombia, where journeys typically begin in the Western Hemisphere.

If the Biden administration’s policies do stay in place and no changes occur as part of the legal challenges, crossings could also eventually start to increase again.

Migrants who are waiting somewhere along their path to the United States could find the danger they face by staying in place, particularly in Mexico, to be so great that they would rather risk crossing the southern border illegally, said Mr. Ernst, the International Crisis Group analyst.

Criminals and cartels prey on vulnerable populations like migrants. Staying in one place makes them targets for forced labor and sex trafficking, Mr. Ernst said.