Story Highlights• Another Iranian-American is arrested in Tehran
• Colleague says his pregnant wife also briefly was detained
• Ex-lawmaker on another detainee: "We are just trying every lever" to free scholar
• U.S., Iranian diplomats are set to meet next week in Baghdad
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WASHINGTON (CNN) -- Iran recently imprisoned a fourth person of dual Iranian and American citizenship, the man's family and colleagues told CNN on Wednesday
Kian Tajbakhsh, 45, an independent consultant and urban planner, was arrested May 11 in Tehran along with his wife, who is pregnant, according to Karim Sadjadpour of the Carnegie Endowment.
His wife was kept overnight and released, but Tajbakhsh is still being interrogated, Sadjadpour said.
Tajbakhsh works with U.S. philanthropist George Soros' Open Society Institute, which confirmed his detention as well. (Learn more about the detained Iranian-American on his Web site)
A network of family friends is developing a strategy to get him released by reaching out to Iranian officials and launching a public campaign, a close family friend said.
The U.S. State Department has confirmed three other Iranian-Americans are in Iranian custody, including scholar Haleh Esfandiari, who was charged this week with conducting activities against the Iranian government.
Ex-congressman: 'We are just trying every lever'
She is being held at Tehran's Evin Prison, which houses many Iranian dissidents and political prisoners.
Former U.S. Rep. Lee Hamilton is leading the effort to get Esfandiari released but admitted Wednesday that "we don't know who to talk to."
"We are just trying every lever we can possibly find," Hamilton told CNN's "American Morning."
"We're doing everything we know how to do, but you don't know where the power setters are in that country."
Esfandiari runs the Middle East program for the Woodrow Wilson International Center for Scholars, headed by Hamilton. After months of house arrest, Esfandiari was imprisoned earlier this month and charged this week with conducting activities against the Iranian government. (Watch jailed scholar's husband reject Iran's accusations )
Hamilton called the charge "an outrageous move" and accused Tehran of "trying to make this very distinguished scholar into a spy."
She went to Tehran to visit her elderly mother in late 2006 and was stopped on December 30 en route to Tehran's airport to catch a flight back to the United States, Hamilton said.
Her taxi was stopped by three men who stole her luggage and handbag, which contained both her passports, according to Hamilton.
While attempting to replace her documents at the passport office, a representative of the Iranian Intelligence Ministry took her for questioning, he said.
According to Hamilton, that began a series of interrogations about her work at the Wilson Center. Interrogators suggested Esfandiari and the institute were involved in "subversive activities."
Hamilton said Esfandiari was allowed to return to her mother's home after each round of questioning, which continued into mid-February.
After not hearing from the Iranian authorities for weeks, Esfandiari was called earlier this month and asked to make what in effect would be a confession, Hamilton said. When she refused, she was summoned to the Ministry of Intelligence last Monday, where, Hamilton said, she was put in a car and taken to the Tehran prison.
Esfandiari's case is similar to that of Parnaz Azima, a female journalist for the U.S.-funded, Persian-language broadcaster Radio Farda, who has been barred from leaving Iran since January.
"These are women who are grandmothers who were there to visit sick relatives," State Department spokesman Sean McCormack said Tuesday. "That's what they were going over there for."
Another Iranian-American held in Iran has not been identified.
In addition, Robert Levinson, an American and retired FBI agent, has been missing since March 8 when he was last seen on Iran's Kish Island. His wife recently traveled to Washington to meet with lawmakers and diplomats who are working on his case.
Pressure from Capitol Hill
All 16 female U.S. senators recently penned a letter to U.N. Secretary-General Ban Ki-moon asking him to intervene in the cases of Esfandiari and Azima.
"The detention of these women is inexplicable and unjustifiable," the senators wrote in the May 18 letter.
"Detaining Dr. Esfandiari and Ms. Azima on questionable grounds contradicts the very essence of their work: to promote peace, reconciliation and freedom for all.
"We urge you to intervene with the government of Iran on behalf of these two Iranian-American women in order to end their unjust detention and ensure their imminent release and safe passage home."
While the State Department continues to pressure Tehran about Levinson and the detained Iranian-Americans, McCormack said the issue will not be discussed next Monday when U.S. diplomats meet with their Iranian counterparts in Baghdad.
"The issue at hand in the meeting between [U.S. Ambassador] Ryan Crocker and the Iranian representative ... is going to be focused on Iraq and stabilizing Iraq," McCormack said at Wednesday's daily briefing.
McCormack was asked whether he believes Tehran is trying to use Esfandiari or any of the other detained Iranian-Americans as leverage to free five Iranians who were captured in northern Iraq in January.
"I haven't seen anybody suggest such a linkage," he responded. "I can't tell you why they're harassing this woman." The five Iranians were detained January 11 in Irbil, an Iraqi Kurdish city near the Iranian border.
They are suspected of having connections to Iran's Revolutionary Guard Quds Force, which the United States accuses of providing weapons and funding to Shiite insurgent groups in Iraq.
Iran complained to the United Nations that the raid was carried out in "clear violation of international conventions" because it took place at an Iranian consulate.
The United States maintains its forces raided an Iranian liaison office that does not have the same diplomatic status as a consulate.
The Geneva Conventions state that embassies and foreign missions in countries are immune from search, arrest and detention.
Secretary of State Condoleezza Rice has said the United States is reviewing the status of the five men through a "normal process."
CNN's Elise Labott and Jill Dougherty contributed to this report.