(CNN) In the last three weeks, two US citizens have been detained by the North Korean regime.

Both men work at Pyongyang University of Science and Technology, one of the few schools to employ foreign experts in North Korea.

Kim Hak-song, an ethnic Korean, born in Jilin, China, and educated at a university in California, was detained Saturday on suspicion of "hostile acts" against the regime , state media reported.

In April, Kim Sang Duk, also known as Tony Kim, was also detained for "hostile acts" while trying to fly out of Pyongyang International Airport.

Michael Madden, a visiting scholar at the US-Korea Institute at Johns Hopkins University, says the timing of the US detention was "no accident."

The detentions come amid heightening tensions on the Korean Peninsula; just last week, North Korea state media published an 1,800-word report accusing the US and South Korea of plotting to kill leader Kim Jong Un.

Friday's report said that a "hideous terrorist group" conspired with the CIA and South Korea's National Intelligence Service to mount the "bio-chemical" attack.

CNN was not able to independently corroborate the report and South Korea's intelligence service told CNN they knew nothing about an alleged plot.

Keeping close watch

Pyongyang University of Science and Technology, or PUST, is run by evangelical Christians. Due to exposure to foreign faculty members, pupils are heavily vetted for something Madden calls "political reliability" before they are admitted.

The university is still highly outward-looking by North Korean standards, in a nation where foreign academics are relatively rare.

A graduation image taken in March 2017 from the Pyongyang University of Science and Technology.

Even so, it's expected that students report on their interactions with foreign staff.

"Foreign nationals in the DPRK really have to allow for there to be no ambiguity or interpretation about any of their interactions or what they're doing," says Madden, a contributor to North Korean monitoring organization 38 North.

He says that plausibly innocuous exchanges can be interpreted differently by the North Korean security authorities who read the reports.

Author goes undercover

In 2011, writer Suki Kim went undercover at PUST, posing as an English teacher and missionary.

Americans detained in North Korea Currently held: • Kim Dong Chul, the president of a company involved in international trade and hotel services, was arrested in 2015 and is serving 10 years on espionage charges. • Otto Warmbier, a University of Virginia student, was sentenced to 15 years of hard labor in 2016 for removing a political sign. • Kim Sang Duk, also known as Tony Kim, a university professor, was detained in Pyongyang in 2017 and accused of attempting to overthrow the government. • Kim Hak-song, a native Korean born in China (Jin Xue Song is the Chinese version of his name) and professor working at the same university as Tony Kim was detained May 6 on suspicion of "hostile acts" against the regime. Americans released in 2014: • Kenneth Bae served nearly two years of hard labor after accusations he was part of a Christian plot to overthrow the regime.

• Matthew Todd Miller was also accused of "hostile acts" after tearing up his tourist visa and seeking asylum after entering North Korea.

• Jeffrey Fowle spent five months in a North Korean prison after being caught with a Bible inside the country.

The subsequent book -- Without You, There Is No Us -- detailed, in Kim's words, "the psychology of North Korea's future leaders and their very complex and human and inhumane world."

"There, in that relentless vacuum, nothing moved. No news came in or out. No phone calls to or from anyone. No emails, no letters, no ideas not prescribed by the regime," Kim wrote in her bestselling book, which was published in 2014.

According to Madden, the behavior of faculty members and exchanges at the university will inevitably have been under additional scrutiny by security forces as a result of the book.

"PUST is certainly going to be in the crosshairs for -- let's say special attention -- by the North Korean authorities because of that book," says Madden.

Suki Kim strenuously denies her book has anything to do with the latest detentions. She told CNN she was "threatened repeatedly" when North Korean security services found out about the book soon after it was published.

She says since then PUST has been able to continue functioning -- and even expand -- under the regime.

The school offers free education to children of the North Korean elite, many of whom would otherwise be sent abroad for school.

"PUST exposes them to all sorts of subject matter and to interesting people," Madden says. "They benefit enormously from that, and North Korea is not going to upset that applecart."

"The book has been out for a while," says John Delury, associate professor at the Yonsei University Graduate School of International Studies in Seoul.

In his view, it's impossible to separate the link with PUST in these detentions from wider tensions on the peninsular.

"So much is at play now and they're both US citizens -- there's a lot in the air now as far as perceived threats."