NEW YORK -- There are very few people in the United States with more power than an officer of the U.S. Customs and Border Protection. At the U.S. border, one person alone can begin a process that could eventually see a visitor's removal -- voluntary or obligatory -- from the country.

It is a troubling irony for a nation founded on the basis of immigration.

The vast majority of people who enter the U.S. come through the Canadian or Mexican border. But more than one quarter of all travelers to the U.S. arrive by air, which following the events over the past few years has seen a dramatic increase in security. Between the moment one passes through security at the departing airport and the moment one passes through customs and immigration checks at the destination port of entry, there is an "airside" environment that still mystifies and fascinates attorneys.

It is an international legal phenomenon that is left much to the discretion of host countries.

The rights that travelers -- citizens or otherwise -- enjoy before properly crossing the border are far fewer than they likely expect.

In some cases, this space between offers travelers far fewer rights than some of the least democratic and free countries on Earth. Limited access to legal counsel, unwarranted searches, and questionable rights to free speech, to name a few.

Chris Eaton, a digital media specialist and environmental activist, is a "one percenter" -- that is, the tiny minority of American citizens that have been, and in many cases will continue to be, subject to additional searches and screening before or after flight into the United States.

According to Department of Homeland Security (DHS) figures, nearly 98 percent of all travelers are admitted to the country directly from initial or "primary" inspection by officials at U.S. ports of entry.

Of an estimated 98.3 million international travelers admitted by air to the U.S. in 2012 (PDF), about 724,000 were detained, arrested, or refused entry to the country, according to data from U.S. Customs and Border Protection (CBP). The reasons are varied: Outstanding foreign arrest warrants, immigration issues, failure to declare the proper documentation, and environmental and agricultural hazards are all cited.

In some cases, no reason is given.

Flying a non-direct flight from the Dominican Republic to Seattle via Miami in early June this year, Eaton said that following a routine search at his departing airport, he was subsequently detained for more than six hours at Miami International Airport, where he had a planned layover.

Nearly 98 percent of land, sea, and air travelers are admitted to the U.S. directly from initial or "primary" inspection by officials at U.S. ports of entry. That said, many millions are not.

Eaton found himself subject to a secondary screening process -- "SSSS" was stamped on his boarding pass -- in which his bags were searched in a separate room. Though officers at the airport were polite and courteous, Eaton suspected that something abnormal was afoot.

He's not alone. Security researcher and programmer Nadim Kobeissi said that he was stopped at least 10 times for secondary screening at U.S. airports between March and August 2012.

He told me via Cryptocat, an open-source Web application he created, designed for encrypted communications, that he believes the "random" additional screening to which he was subject was related more to his ethnicity than his work; in fact, he was once told by airline staff that he could be "on a list." During his airport interrogation, U.S. officials raised questions about his work in software development. On one occasion, his U.S. passport was confiscated and sealed in a plastic bag for more than an hour.

"I was not told of my rights," he said, adding that he did not ask, either.

Pre-flight secondary screening can be random. It can also be targeted based on criteria that alert authorities, such as one-way flight reservations and tickets paid in cash. It can be conducted at a U.S. airport by the Transport Security Administration (TSA), or by a foreign agency of the same authority, often under instruction and guidance from the DHS.

The U.S. government is known to maintain a list of names that are subject to additional screening, from regular citizens to diplomats and politicians. As of December 2009, the list reportedly contained 16,000 names. It is believed that the total has increased by many thousands since.

Only later would it become clear to Eaton, once he was subject to six hours of detention at Miami, that this search was targeted and directed at him, he said.

Detained, searched, interrogated

Eaton says he was asked numerous questions during his U.S. interrogation, including questions about his family, his partner's family, his employer, and details of his prior travels, information which many would consider to be personal and sensitive. Unlike Kobeissi, who was split on his belief of why he was stopped more than a dozen times in the space of a few months, Eaton is "nearly certain" that this unwanted attention from U.S. officials was entirely a result of his work.

Eaton's work as a digital media specialist has a heavy focus on the climate movement. He noted that the officers were "particularly interested" in his trip to Alaska on the Greenpeace ship MV Esperanza.

While he had his suspicions on what the CBP officers were after, "I learned later that when they pulled my partner aside, the first thing they asked was if she had my phone. I think it's pretty clear that is what they were after." He said that the officers wanted his phone so they could see who his contacts were at particular organizations.

But things became heated when the officers asked for his iPhone passcode, in which he noted his discomfort in handing it over.

It was clear to him that the implication was that should he fail to hand over his passcode, he would not be admitted into the U.S. They asked questions laced with accusations, such as whether he was hiding something "like child pornography?" They said they couldn't answer what they would do with his phone should he disclose the code.

In Eaton's case, he asked for legal representation -- and was informed that he had yet to be admitted to the U.S. and therefore did not have right to counsel. That stands in conflict with advice (PDF) offered by the American Civil Liberties Union (ACLU). The border agents implied that Eaton had no rights while he was being detained, he said.

Unaware of his rights, he eventually buckled under the weight of worry for his partner. He said he did not see his phone for the next four hours.

He strongly suspects that his phone was plugged in after noticing all the notes in his iPhone were deleted. The battery had no charge left, he said, so "they had to charge it."

Eaton was released after six hours without being told why. "I was never told why I was being detained," Eaton said. "In fact, I believe I was misled at times [by] being told it was random and at other times being told that I was not allowed to know."

"I was never told why I was being detained ... In fact, I believe I was misled at times ... being told that I was not allowed to know."

He learned that various airline staff and airport police had told his partner they had spoken to customs officials and that he was actually being held by the DHS, and even that there was a warrant out for his arrest in Seattle.

He told me that amid concerns that he could be arrested or even deported, it was "difficult to untangle what was speculation" and what was passed from one officer to another.

He later confirmed, once he landed back in Seattle, that there is and never was a warrant for his arrest. "I have never been arrested."

Weeks later, agents from the U.S. Federal Bureau of Investigation visited Eaton at his apartment. He was one of many several climate activists in the city who had been subject to approaches by the agency, he told me. Seattle-based newspaper The Stranger confirmed that six other activists were approached by the agency in July. Eaton explained that the agents wanted to "clear up" the trouble he had experienced at the airport. After saying he wished to consult a lawyer, he declined to speak to them again.

Where U.S. jurisdiction starts, but not where U.S. legal rights begin

Immigration lawyers and defense attorneys find the border captivating because it is an ill-defined, seemingly stateless space between countries -- that is, a minefield of conflict for domestic, foreign, and international law.

Once a traveler passes through security at the airport, they enter an "airside" transit zone (often studded with duty-free shopping opportunities). These areas remain the jurisdiction of the country in which the airport is based, but "without the constraints that you would normally have within a liberal democratic society," according to Danish researcher and sovereignty expert Thomas Gammeltoft-Hansen, speaking to National Public Radio in July.

This kind of transit zone is where David Miranda, the partner of journalist Glenn Greenwald (who published a string of U.S. government documents leaked by former National Security Agency contractor Edward Snowden), was detained for almost nine hours at London Heathrow Airport earlier in August.

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By invoking Schedule 7 of the Terrorism Act, a statute within United Kingdom law that only applies to a border area and a port of entry such as an airport, U.K. government officials were able to prevent Miranda from contacting an attorney and force him to hand over passwords to his electronic devices. There is no right to silence under the Act.

Miranda, a Brazilian national on his way to Rio de Janeiro (where he resides with Greenwald), said he was "escorted to passport control so he could enter Britain and wait there." By leaving the border area and entering Britain, only then was he afforded rights that U.K. residents enjoy.

A similar kind of situation could arise between the space where a traveler leaves the U.S. via a transit zone, or enters the country at one of the 329 official U.S. ports of entry, defined as a place where one may lawfully enter the U.S., including by land, sea, and airports. In other words, the U.S. "border" is not simply the collection of places where one crosses from Mexico or Canada into the U.S., and the points at which a border or customs official hands back a stamped passport. The reality is far more complicated.

"Legally speaking, and in reality, governments have taken sort of quite a few liberties in creating what you might call sort of smooth spaces," Gammeltoft-Hansen said. "In some situations, including nearby hotels, nearby detention centers -- even a court many miles away from the actual airport and still sort of maintaining this idea -- well, this is part of an airport transit zone."

The practice means millions of travelers who pass through these areas remain subject to U.S. law, but enjoy fewer rights, because they have yet to officially enter the country.

The U.S. government's designation of its "border" can stretch as far as 100 miles inland from the point where it begins, according to the ACLU -- large enough to include about two thirds of the U.S. population, as well as almost every major U.S. city, including New York, Boston, Washington D.C., Chicago, Seattle, Los Angeles, and San Francisco.

This broad definition gives the U.S. Border Patrol, a division similar to U.S. Customs and Border Protection, the "claimed authority" to set up checkpoints on highways, in which "administrative stops" occur and drivers and passengers are often asked common questions, such as their nationality.

Pray for a "primary": Stamp, and you're in

U.S. Customs and Border Patrol (CBP) officers stationed at ports of entry at the U.S. border are charged with detecting the importation of illegal and dangerous substances and "enforcing laws controlling the flow of persons and goods across the borders," according to a DHS document from late 2011 (PDF).

Homeland Security provides CBP officers access to criminal records and other information to determine a person's eligibility to enter the country. These details are often given by foreign nations under existing legal agreements -- known nation to nation as "mutual legal assistance" -- or through wider policies, such as the European Union's "passenger name records" treaty.

Most people experience what is known as a "primary inspection," where a CBP officer examines a visitor's or citizen's passport and asks questions to establish whether they are admissible to the United States.

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Scott Railton, an immigration lawyer at Cascadia Cross-Border Law, told me on the phone that issues arising at the border are mostly split into two major categories: Customs violations or immigration violations. However, the most common issues are agriculture related, such as bringing in goods that may disrupt the U.S. farming ecosystem or environment, like plants or foodstuffs.

These are often dealt with at the border, where fines can be imposed upon the failure to disclose certain goods.

"The border is mostly split into customs violations and immigration violations," he added. "Just about any kind of narcotics offense makes a visitor inadmissible to the U.S. The narcotics provisions of the Immigration and Nationalities Act go further than just convictions. For instance, if the border officer has a 'reasonable suspicion' that a person has engaged in drug trafficking in the past, they don't need to show proof of conviction to deny somebody admission to the U.S."

In any case, if a border official suspects that something is amiss, they can and will refer the subject to "secondary inspection." These remaining people will receive on their declaration forms an "A" for a check on food items, a "B" for an immigration issue, or a "C" for a further inspection of luggage.

Railton, whose law firm is based close to the Canadian border, explained that the law defines convictions so broadly that they can include what may have been a conviction at one time. "Even if a crime was later expunged, for immigration purposes they still persist. For example, a pardon in Canada is not recognized in the U.S. for the purposes of immigration."

By law, U.S. citizens cannot be rejected from entry, but they still have to demonstrate or prove in some way -- such as by presenting a valid passport -- that they should be allowed back into the country.

"The burden of proof is on the person to show they are admissible to the United States," Railton said.

Border tech, electronics searches: What rights do you have?

The few rights that are afforded to Americans and visitors alike at U.S. ports of entry are by far outweighed by the authority of the border officials.

One of the more controversial -- and yet still a legally contested gray area -- are the rights that travelers have in regards to electronics and device searches. Where visitors and citizens alike are afforded constitutional rights of free speech and expression and the right to protection against unreasonable searches and seizures while in the United States, at the border many of those rights do not exist in full.

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Luggage and electronic device searches while within the U.S. would ordinarily fall under the Fourth Amendment, which prohibits warrantless searches without probable cause. But a search warrant is not needed to search a device at a port of entry.

Railton noted that rights under the Fourth Amendment are not completely suspended at the border, at least as a matter of law. "The long-standing rule is that the border searches are 'reasonable' by virtue of the fact that they occur at the border," he said.

According to documents obtained by the ACLU as a result of a Freedom of Information Act request, more than 6,600 travelers -- nearly half of whom are U.S. citizens -- were subjected to electronic device searches at U.S. ports of entry between October 2008 and June 2010.

In the 2011 DHS document, the CBP says it can conduct a "brief physical inspection," such as switching a device on, to demonstrate that it is not a container for illegal substances, but it has the right to "search ... the device's contents." A device can also be detained, which can result in a copy of the data contained on the device being stored for later forensic searches. And a copy of the data can be used for "evidence of continued or future admissibility." Device detentions are often returned within seven days, but can be extended by up to 14 days, the document says.

Earlier this year, a U.S. federal appeals court ruled that a device can be searched if border officials have "reasonable suspicion" of criminal activity. This notion of "reasonable suspicion" is a legal right held by law enforcement to temporarily detain a suspect under the condition that they have committed or could commit a crime, though it's different from "probable cause," which is a higher level of suspicion that can justify a warrant. CBP officers are trained to detect suspicious or nervous activity at a U.S. port of entry, which can be used to justify grounds of reasonable suspicion.

Border officials can, if they have difficulty searching the device due to translation matters or encryption issues, ask for outside agency help. The EFF notes that it is unclear on whether cooperating agencies can keep hold of the data indefinitely.

There are changes afoot in the courts, which may lead to changes in how electronic devices are searched at the U.S. border. A petition to take the case to the U.S. Supreme Court was filed in early August, which will add further clarity to the case.

A copy of the data taken from smartphones and laptops can be used for "evidence of continued or future admissibility," the CBP says.

Railton explained that a person does have the right to silence under the Fifth Amendment, where someone may refuse to answer questions, but this is rarely in the favor of the visitor seeking entry to the U.S.

Conversely, the constitutional rights that govern free speech and expression are also arguably diminished at the border.

According to the ACLU, official CBP policies bar Americans and visitors alike from recording video or sound without prior approval from a senior port of entry official. This violates the First Amendment right to document the public operations of law enforcement agencies, the privacy group warned, and sued the government to that effect.

But documents published by The New York Times earlier in August showed that the DHS is spending $5.1 million on a system dubbed "BOSS" -- the Biometric Optical Surveillance System -- after two years of taxpayer-funded development. The report says the system would help match faces in a crowd with names on a "watch list," whether in searching for suspected terrorists at high-profile and crowded events, or looking for fugitives in busy, populated areas.

The technology relies on a vast database of faces, the majority of which the DHS says are collected at border crossings (PDF) by the CBP.

You're in. But is the damage already done?

Though Eaton managed to successfully enter the U.S., he was angered by the hostile attitude that his government increasingly seems to take toward its own citizens.

"It feels like the federal government can now harass whomever it wants at airports," he said. "There was no good reason to detain me. This allows them to confiscate electronics which they mine for contacts and other data, and further harass and intimidate whomever they want.

"No matter how small, these events have deep impacts on our psyches. The authorities disempower and confuse us with such practices, making it hard to live your own life instead of the one that they want."

We reached out to the CBP for comment during the writing of this article, but did not receive a response.