Four high-level officials of a South Korean umbrella labor union were arrested Tuesday, in the latest development in a widening investigation into suspected North Korean espionage.

In a statement Tuesday, investigative and intelligence authorities said they found “substantial evidence” to suspect the labor union officials of working with North Korean spies.

In January, police and the intelligence service together searched the homes and offices of the four labor union officials and found more than 100 documents suggesting they were communicating with North Korea and aiding its spying activities, the statement read.

The latest suspicions brought under scrutiny the labor union’s history of protesting against South Korea’s joint military exercises with the US and the US forces being stationed here.

Speaking to reporters Tuesday, the ruling party's spokesperson Rep. Kim Mi-ae said that the labor union’s anti-US strikes “do nothing to improve labor initiatives.”

She added that the arrest warrant being issued for the labor union officials signifies the “court acknowledging the gravity of the suspected espionage crimes.”

Rep. Tae Yong-ho, who sits on the ruling party’s supreme council, worried that the labor union’s anti-US forces rallies could potentially jeopardize South Korea’s alliance with the US.

“Imagine if these images, with no context, were to be presented to an unknowing third party as if they represent the South Korean majority,” he said in a press conference last month.

So far the labor union has denied involvement in North Korean espionage, with its spokesperson telling The Korea Herald that the investigations appeared to be “an attempt to paint the union as pro-communist.”

“It’s an old-time ploy,” he said.

The labor union is just one among several South Korean groups under investigation in the latest spying scandal.

Two weeks ago, four members of a "Korea unification group" were indicted for being in a North Korean spy ring. According to Seoul prosecutors, the group’s four members had regularly met with North Korean agents in Southeast Asian countries like Cambodia and Vietnam over the years, colluding with them on spying and other operations.

The head of the left-leaning Progressive Party’s committee on Jeju Island, Park Hyun-woo, was arrested last month after allegedly praising the Kim Jong-un regime.

President Yoon Suk Yeol has on multiple occasions expressed that he considers the emerging suspicions of North Korean espionage-related crimes to be serious.

During his first dinner with the new ruling party leaders elected earlier this month, the president “showed deep concerns” over a series of revelations in the suspected spying scandal, party sources said.