Theodore Roosevelt Island is small and peaceful. It sits in the Potomac River in Washington, D.C. The island is a tree-filled memorial to America's 26th president. It includes several trails and a huge statue of Theodore "Teddy" Roosevelt. The island is one of more than 400 sites within the U.S. National Park Service.

On most days, local people visit the island to escape city life. They walk or run its forested paths. Tourists also visit the island to see the memorial that honors Teddy Roosevelt, known as the "conservationist" president. They often take photos of themselves with the statue.

But, on a recent Monday morning, more than 40 people from 28 countries arrived at the island for a different reason. Instead of exploring, they recited an oath of allegiance. Instead of just taking pictures of the Roosevelt memorial, they sat in its shadow, holding American flags.

On that morning, they became citizens of the United States.

"I'd also like to thank the National Park Service for hosting us here at Theodore Roosevelt Island..."

That is Sarah Taylor. She is the Washington district director of the United States Citizenship and Immigration Services, or USCIS. It is celebrating the National Park Service's 100th anniversary this year by holding citizenship ceremonies in national park sites across the country. The agency has already done more than the 100 ceremonies it had planned.

In August, for example, 15 people became American citizens at the Grand Canyon. In September, 37 people gained their citizenship at Yellowstone National Park.

The naturalization ceremony at Theodore Roosevelt Island was the 13th such event in the Washington, D.C. area this year.

Alexcy Romero is superintendent of the George Washington Memorial Parkway. The National Park Service protects this historic road and the scenic area around it. Romero spoke at the ceremony. It was his first time attending a naturalization ceremony.

"This ceremony is really very fitting, having it in a national park, our national parks: people know them as their natural beauties, those scenic views, that are breathtaking, but we also have incredible sights that tell the American story. These naturalization ceremonies are a good piece - integrating their citizenship with our national parks, where they can learn and understand our democracy and how we came to be who we are today."

The ceremonies mark the end of an immigrant's effort to gain citizenship. But they also mark a new beginning.

The naturalization process

"Naturalization" is the legal act of becoming a citizen. In general, a person can become a naturalized American citizen if they are over the age of 18 and if they have lived in the U.S. for five years, without leaving the country for more than 30 months.

The naturalization process has several steps. A person must first take a test in English on American civics. They must also prove they are of good character. To do that, two American citizens must verify that the nominee will be a good and loyal citizen of the United States.

The process also includes completing paperwork and interviews.

The final step is to take an oath of allegiance to the country at an official naturalization ceremony.

At Theodore Roosevelt Island, Sarah Taylor with USCIS led the crowd in the oath of allegiance.

...and that I take this obligation freely, without any mental reservation or purpose of evasion; so help me God. Congratulations, you are America's newest citizens!"

During the last 10 years, more than 6.6 million people have become naturalized American citizens. Last year, more than 700,000 people gained their citizenship.

Those who became naturalized citizens at Theodore Roosevelt Island came from 28 countries, including Vietnam, Turkey, Pakistan, Venezuela and the Philippines.

For some, the path to citizenship was a long one. Moid Ali works at a bank in northern Virginia. He is originally from Pakistan.

"I came here in January 2012. And, my wife is a U.S. citizen. So I moved here with her."

Ali called his path to citizenship a "rigorous process."

"It's been a rigorous process, as far as like six or seven years back. So, multiple documentations and interviews, getting my Green Card, and applying for the citizenship."

After the ceremony, he felt relieved.

"I'm relaxed. Excited. Definitely an overwhelming experience, you know, being a part of the whole ceremony."

Aaron Gaza said he wanted to become a citizen to serve the nation in the U.S. Navy. He came to America from the Philippines three years ago. He joined the Navy in January.

To become a U.S. citizen for me is very important, to be able to do my job in the Navy, in the U.S. Navy."

Gaza was chosen to lead the rest of the group in the pledge of allegiance.

"I pledge allegiance to the flag of the United States of America. and to the republic for which it stands, one nation under God..."

Zainab Bangura left her native Sierra Leone to continue her education in America. She arrived five years ago.

"There was a certain time in my country when we had the civil war and rebels were all over. And education-wise was like, going down the drain, we don't have enough infrastructure. And like, coming to America, I see this as an opportunity to expand and further my education."

Bangura said becoming a U.S. citizen was an easy process for her.

"It was pretty much smooth for me. I went through it, I registered, and it was exciting. It takes like three months."

Bangura and other newly naturalized citizens now have all the liberties and rights of a full American citizen. The only real limit she faces? She cannot become the president or vice president of the United States.

But she and other naturalized citizens across the country will play a part in electing the country's next leader.

New citizens and new voters

The citizenship ceremony at Theodore Roosevelt Island took place on the final day to register to vote in nearby Virginia. New citizens who live in the state lined up at a small tent near the site of the ceremony. Workers there helped them register.

For Maria Sifontes, the ability to vote [in the upcoming U.S. elections] was a major reason why she wanted to become a citizen this year. Sifontes is from Venezuela. She came to the U.S. in 2007 to study English. She stayed in the country to study criminal justice. Sifontes now works in the Washington area as a lawyer.

"I decided to become a citizen because I really wanted to vote this year. I feel that we need to be more involved in the democracy of the country. We work hard, and I think it's fair enough to participate and that our voice can be heard."

Park Superintendent Alexcy Romero said he was honored to watch 42 new Americans gain their citizenship.

"Just sitting there watching their expressions as they were being sworn in, wondering what was going on through their mind and ... the struggles to get to this point, the struggles of leaving their country and to become an American citizen. It's just moving. It was just one of those events that I was honored to share it with them."

I'm Ashley Thompson.

And I'm Caty Weaver.

Ashley Thompson wrote this story. Caty Weaver was the editor.