(CNN) Three years after chants of "build that wall" became a rallying cry for the candidacy of Donald Trump, his administration is engaged in an increasingly aggressive land grab along the Southwest border to make a new wall a reality, a CNN review of federal court filings shows.

Through November 15, the Trump administration had filed 29 eminent domain suits tied to border-wall construction this year, up from 11 each of the past two years, according to federal court records. All but four of this year's suits were filed in Texas. Eminent domain is the right of a government to seize private land for public use, while providing compensation.

Last month, in the Rio Grande Valley, construction began on the first new barriers along the border since Trump took office. US Customs and Border Protection expects to build about three miles of new wall over the next few months, an agency official recently told CNN. Mark Morgan, CBP's acting commissioner, has said the agency wants to build 450 miles of new wall by the end of 2020.

Approximately 1,300 miles of the 1,950 mile US-Mexico border do not have fencing; these areas often are treacherous terrain or privately owned.

A conservation group, Friends of the Wildlife Corridor, is among the landowners now in court. The US government filed an eminent domain suit in July over a 72-acre parcel the group owns on the banks of the Rio Grande. For more than two decades, the group has worked to try to create a wildlife corridor along the winding river, and its members had planned to sell the 72 acres to the US Fish and Wildlife Service to connect two existing wildlife refuges, said board member and attorney Paul Gaytan. But the new border wall has bulldozed that effort.

"Unfortunately, the bottom line with this litigation is the government has a right to take the land," said Gaytan. "The only question is: What's fair compensation? What's the value of that land?... How do you put a price on this native habitat and the purpose for which it was to be used?"

Federal law requires the government to offer "just compensation" based on fair market value, typically established through appraisals. In some suits, the disputes are based on competing appraisals and disagreements over the market value or on the likely impact of the wall on the land's value, attorneys said. In Cameron County, for example, the government has offered landowners from $14,775 an acre to $67,405 an acre, according to federal court filings in current cases.

The eminent domain suits are the tip of a much broader effort to take private land, say attorneys representing landowners.

"The lawsuits are only a fraction of the condemnations that have been settled outside of litigation," said Ricky Garza, a staff attorney at the Texas Civil Rights Project, which is representing some landowners.

Since 2017, the US government has also filed at least 35 notices -- called "declarations of taking" -- in Starr and Cameron counties, confirming that federal authorities have acquired land or easements to property through the process of eminent domain.

While CBP leaders have called acquiring land "a challenge," neither CBP nor the Department of Justice responded to repeated CNN requests for comment on the lawsuits or on condemnations settled out of court.

Internal CBP emails, obtained by the Sierra Club through a Freedom of Information Act request and shared with CNN, show that in February 2018 the agency identified more than 1,100 land parcels of interest along the path of the proposed border wall in South Texas.

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The process typically begins with CBP sending so-called "right of entry" letters asking landowners voluntarily to grant the government the right to access their land to conduct surveying and soil testing as a prerequisite to possibly taking the land, said Garza.

The Trump administration has sent letters to dozens of private landowners in Texas and other border states in order to survey their land for future border barriers, according to two US defense officials.

"The Right of Entry letter grants the government permission to enter specified private lands to conduct environmental assessments, property surveys, appraisals, geotechnical, and other exploratory work to facilitate future land acquisition and construction of a border barrier on those lands," US Army spokesperson Cheryle Rivas told CNN in an email.

The Texas Civil Rights Project has run radio and newspaper ads and sent out mass mailings in various places along the border to try to reach landowners who may be receiving the letters, Garza said.

"If you don't answer the letter, you get a home visit from a Border Patrol vehicle, pulling up to the house with an officer with the Army Corps of Engineers, an armed Border Patrol agent and often someone from the Department of Justice as well," he said. Several landowners described similar visits to CNN.

Karla Vargas, a senior attorney at the Texas Civil Rights Project, said many landowners don't understand they have a right not to sign the request for entry.

Nayda Alvarez, a schoolteacher in Rio Grande City, is one landowner who refused to sign. She said she has just under an acre of land, adjoining a more than six-acre parcel still in her grandfather's name. She is adamantly opposed to the new barriers.

"Once, maybe twice, they called saying they wanted to come in and survey. It (the letter) also said they could ingress and egress into any property I had, and I said, 'No, I'm not signing; I don't like the wording,'" she said.

CNN spoke to several landowners and attorneys who gave similar accounts of the right-of-entry letter , which requests temporary access to privately owned land but doesn't disclose the consequences of not doing so.

"People don't know the reality of it and the severity of what's going on," Alvarez said. "They waived environmental laws that will affect us; they even waived the Clean Water Act. The river is right there. Where are we going to get our clean water from?"

In October 2018, then-DHS Secretary Kirstjen Nielsen waived more than two dozen laws to expedite border wall construction in Texas, including the Endangered Species Act, the Clean Air Act, the Clean Water Act, the Safe Drinking Water Act and the Migratory Bird Conservation Act, among others.

As of Tuesday, the government hadn't filed an eminent domain suit against Alvarez's property.

Members of the Cavazos family, which own 64 acres near Mission, Texas, were sued by the government for their land on November 5, 2018. They are worried about losing a big chunk of that land, roughly 27 acres, to wall construction, said Baudilia Cavazos Rodriguez.

The land has been in her family since the 1950s, she said. Her brother and sister have more than 30 renters living on their part of the property.

"My grandmother bought the land; she had my dad and uncle sign for it. After World War II, we planted cotton and had cattle, all kinds of vegetables, we had a boat ramp," she said. "We're devastated because this is something my grandmother worked for her whole life."

She said her siblings rely on the rent, and "make only about $30,000 a year. My sister Eloisa is a retired teacher, my brother is handicapped in a wheelchair. If they put in a wall, we're thinking those renters will want to leave," said Cavazos.

On October 10, after 11 months of legal wrangling, the government agreed to pay the Cavazos family $350 for access to survey their property, instead of the $100 it originally offered. As of Tuesday, the case hadn't moved to the acquisition phase.

There is relatively little dispute over the government's right to use eminent domain to acquire private property, especially when it cites national security as a justification, said Texas Civil Rights Project staff attorney Emma Hilbert.

"A lot of times, delay is the name of the game," for the landowners, she said. With a presidential election next year, there's always the chance that a change in administration could mean a change in border policy. In any event, Hilbert said, delay "is good for them as landowners, regardless of what the administration looks like now or a year from now."

Scott Nicol, a longtime Sierra Club volunteer in South Texas, said the first round of border wall construction just over a decade ago offers an important lesson for landowners now facing land seizures.

"When the walls were built under the (George W.) Bush administration, if you took the first offer, you came out really bad. The landowners that fought invariably got more money," he said.

A CNN analysis of 442 lawsuits from that earlier round of border wall construction found that property owners often were offered much less than their land was worth and that at least one-fourth of those who challenged the government's initial offers received more money. That figure doesn't include out-of-court settlements. Some of the suits also stalled construction at times, CNN found.

"Land acquisition is going to continue to be a challenge," Morgan, the acting CBP commissioner, said at a November 14 news conference, "You could have a mile of land -- again, on the southwest border -- where it goes back in time, and you could have multiple owners, from 10 to 100 owners, that have a piece of that land. Sometimes the records go way back; the records weren't that great. And it's a challenge to go through that process ... but again, I still think that we're on track to get the land we need for 450 miles."

Jim Chapman, vice president of the Friends of the Wildlife Corridor, said his group planned to fight from the moment they knew the government had its sights on their property.

"While a lot of people are consenting because they don't see any way to fight it, you can certainly slow the process down; that's what we're doing," he said.

Chapman said that because of agricultural development, very little of the brush and natural habitat along the river remains. Those few areas become magnets for hundreds of migrating bird and butterfly species and those that live in the Rio Grande Valley year-round, along with endangered animals such as ocelots and jaguarundi.

"What is left is extremely valuable," he said. "That's why saving 72-acre parcels matters, because there's so little of it left."