(CNN) Lawmakers on Capitol Hill couldn't figure out over this past summer why nearly $400 million in aid they'd voted to go to Ukraine still wasn't in the country's coffers.

There was growing speculation by the end of August. Congressional leaders, their aides and members of key committees -- including the Appropriations, Armed Services and Foreign Relations committees -- were scrambling to figure out why money that had been appropriated by Congress months before still hadn't been disbursed. Outreach by lawmakers to key agencies left few clues other than the delay was coming from the White House and no one could pinpoint exactly what the reason was.

Senate Majority Leader Mitch McConnell got involved, reaching out to both Secretary of State Mike Pompeo and Defense Secretary Mark Esper as other members urged action with letters, public statements, floor speeches and staff outreach.

"I have no idea what precipitated the delay, but I was among those advocating that we needed to stick with our Ukrainian friends," McConnell, a Republican from Kentucky, said about his outreach last week.

The comment came in a week in which a redacted, whistleblower complaint was released alleging that Trump was trying to elicit dirt from Ukraine's President Volodymyr Zelensky about his political rival, Vice President Joe Biden (There is no evidence of wrongdoing by either Joe Biden or his son, Hunter). Earlier this month, the White House also released a transcript a July phone call between Trump and Zelensky in which Trump touts the US support for Ukraine while Trump later asks Zelensky for a favor.

In more than a dozen interviews with members of Congress and aides in recent days, many lawmakers on both sides of the aisle say that while they don't remember the exact timeline of when they became aware there was an issue with Ukraine funding, there was a growing sense at the end of August that Congress needed to push the administration harder when it returned from its Augusts recess to release it. Sen. Lindsey Graham, a member of the Senate Armed Services and Foreign Relations Committee, told CNN he talked to the Pentagon about the funding multiple times.

"I called the Pentagon. The Pentagon said that they were worried about the new administration, they were doing their due diligence, they were worried about corruption, they were worried about military aid. They wanted to figure out what was what. I said, 'Fine, just figure it out,' " Graham said, adding that it wasn't unusual for President Donald Trump not to be keen on foreign aid.

"As to the President, he wants to withhold aid across the board to get people to pay more," said Graham, a South Carolina Republican and key congressional ally to Trump, arguing there is no connection between the money and the President trying to get dirt on his potential political opponents.

Trump and his supporters on Capitol Hill and at the White House have denied there was any quid pro quo . Instead, Trump's supporters have argued the delay in military funding came out of a fear the administration had about corruption in Ukraine.

Over the summer, Sen. Bob Menendez, the top Democrat on the Senate Foreign Relations Committee, had his staff making regular phone calls to the State Department while Menendez himself applied public pressure.

In a statement on August 29, Menendez said "in willfully delaying these funds, the Trump Administration is once again trying to circumvent Congress' Constitutional prerogative of appropriating funds for U.S. government agencies. It is also undermining a key policy priority that has broad and deep bipartisan support."

Sen. Chris Murphy, a Democrat from Connecticut, who planned to travel to Ukraine in the first week of September also released a statement urging the administration to release the money.

"Everything I had heard was that this was a decision made by the President. It was his decision and it was his decision alone," Chris Murphy, a Democrat from Connecticut, told CNN.

Republican Sens. Ron Johnson of Wisconsin and Rob Portman of Ohio each spoke directly with the President about it as well.

Earlier in the summer, most members and aides hadn't raised a red flag on Ukraine aid. The money hadn't been sent out, but given this was an administration where key players had made no secret of how they felt about foreign aid, it wasn't particularly unusual.

The President had been open he detested how much the US government sent to countries abroad. It was a refrain of his campaign. And Mick Mulvaney, the President's acting chief of staff and OMB director, who had earned a reputation from his colleagues on the Hill as a take-no-prisoners budget slasher, had been pushing for a rescissions package throughout the summer that would have cut roughly $4 billion in foreign aid. Many members and aides assumed a holdup on Ukraine funding could have something to do with the package.

On August 22, news reports indicate Mulvaney and the administration had relented on rescissions, however. And, it was after that fight was over, members began to wonder why military aid to Ukraine still had not gone out. At the end of August, Politico reported millions in military aid to Ukraine was still being slow-walked.

A bipartisan priority

Lawmakers were struck by the fact that this was money that had already been appropriated with broad bipartisan support. Ukraine military aid was a rare, foreign policy issue that united members of both parties. Supporting the country was widely viewed on Capitol Hill as a way to deter Russian aggression, keep them at bay and secure the region. The fact that the money was being held up and without a clear explanation or briefings about a changing policy prescription in the region, bothered many.

While, it wasn't unprecedented for an administration to change course or hold back money if there was an evolving situation in a country they were aiding, it was unusual for members of the relevant committees not to be in the loop on those discussions.

"We would have expected congressional notification that in fact monies were being held, and that would have elicited from us a briefing and normally we would have gotten a briefing," Menendez told CNN.

He added, "This is emblematic of this administration in so many ways."

There were two separate lines of money being held up. There was $250 million in military aid that was appropriated to come from the Department of Defense that had yet to be dispersed and another $140 million that was supposed to come from the State Department. A Democratic Senate aide told CNN last week that the Defense Committees had been alerted by the Department of Defense that they were prepared to send off $125 million in February and then another $125 million in May.

A top Pentagon official sent a letter to Congress in May certifying Ukraine was making progress in the fight against corruption justifying the US provide Ukraine with a $250 million military assistance.

On June 18, the Pentagon announced plans to provide $250 million to Ukraine in security cooperation funds for additional training, equipment and advisory efforts to build the capacity of Ukraine's armed forces.

Then, nothing, according to a congressional aide and a US official familiar with the correspondence.

"We'd been given signals twice by the administration that they were going to release the funds then nothing happened. In August we were told the OMB is holding it," Senate Minority Whip Dick Durbin said. "They were withholding these funds that had been appropriated and signed into law by the President until the last two weeks of the fiscal year. That's crazy. It hardly ever happens,"

Another Democratic aide told CNN that the State Department notified OMB of their intention to obligate the $141 million in aid to Ukraine on June 21. Typically, that process would have just been a courtesy. OMB would have had up to five days to ask questions about the process. But, instead, it was more than two months before Congress received a notice that the money was being dispersed.

State Department officials told Senate staffers in a briefing last week that the department had no objections to the money moving forward and were not aware that OMB had sat on it, according to a Senate source. But officials in the meeting pointed the finger directly at Mulvaney as the person who directed the State Department not to send out the funds.

In the beginning of September, Johnson and Murphy traveled to Ukraine. When they returned, they both talked to Durbin, an Illinois Democrat and vice chairman of the Senate Appropriations Defense Subcommittee, about the need to make sure Ukraine got their money.

"I will tell you who brought it up to me was Ron Johnson," Durbin said. "He came back from Ukraine and said 'we gotta get this money released.' And, I said, 'I'll look into it.' "

Murphy would say later about his time in Ukraine that there had been "near panic" in the country during his trip about whether America was really committed to their relationship in part because of delayed aid.

In the meantime, more senators were catching on to the fact that the money was being held and they pointed their attention at Mulvaney. On September 3, Portman and Johnson along with Democratic Sens. Jeanne Shaheen of New Hampshire and Richard Blumenthal also of Connecticut -- members of the bipartisan Ukraine caucus -- sent a letter to Mulvaney demanding answers as to what was going on with the money.

"This body has long advocated for increasing the military capacity and capabilities of Ukraine — a fledgling democracy that is pro-West and pro-United States and since 2014 has been under increased military, political and economic pressure from Russia," the senators wrote.

In the House, the Foreign Affairs Committee Chairman Eliot Engel of New York and the committee's top Republican, Rep. Michael McCaul of Texas, sent their own letter to Mulvaney two days later.

Finally, there was an opportunity to put the Trump administration on the spot.

A little-noticed committee vote

Durbin had been working on an amendment, which would hold back money to the Pentagon if the administration didn't spend military aid money for Ukraine on time. He'd been talking with one very concerned Republican about it: Lindsey Graham.

On September 12, the Senate Appropriations Committee gaveled into their Defense Appropriations markup hearing. The routine session was mired with fireworks over the President's campaign promise of a border wall between the US and Mexico, but the meeting made news for another reason.

Graham announced the Trump administration had finally released the military aid for Ukraine. There was no longer any reason for Durbin to offer his amendment.

"Why was it released? Because of your amendment," Graham told the committee. "That is why it was released because I was going to vote for it ... If you are listening in Ukraine on C-SPAN, you are going to get the money."

Graham pushed for Durbin to drop his amendment.

"We can call the Secretary of Defense at any time we want to about 2020 funds and say, 'When are you going to release the funds to the Ukraine? If you have a legitimate problem, tell us now.' If they give us the run-around, count me in on a bill that will get 90 votes," Graham said.

Durbin spoke anyway, flirting with the idea he'd still force the committee to vote on his amendment that would put pressure on the administration to spend Ukraine aid money in the next fiscal year or face consequences.

But, multiple Republicans also chimed in that they were concerned about the money having been withheld. Sen. Susan Collins, a Republican from Maine, said she was "inclined" to vote for the amendment because "I think it has been a problem that the money has not been released."

Louisiana GOP Sen. John Kennedy asked multiple times, "Mr. Chairman I would like to ask Sen. Durbin and Sen. Murphy why they think the funds were held up. "

Durbin responded, "We don't know."

Durbin withdrew the amendment, which he said at the time was an act of good faith.