Washington (CNN) While the US killing of Qasem Soleimani and ongoing delay in a Senate impeachment trial of President Donald Trump are taking center stage, there's a major battle in the courts and federal agency offices to pry loose information about Trump and Ukraine the White House has tried to keep secret.

Congress has faced an unprecedented blockade from interviewing key witnesses and seeing key documents during the impeachment inquiry, leading to the US House's to include obstruction of Congress as one of its impeachment articles. But news organizations and transparency groups have picked up the effort to pry loose memos, emails, calendars and other documents from the Trump administration.

Those efforts are already paying off, creating a surprising twist before Trump's eventual Senate trial.

Previously unseen Trump administration emails obtained by news outlets and transparency groups exposed the administration's scramble to withhold nearly $400 million in US military assistance from Ukraine, and further pinpoint Trump's role in ordering the freeze.

The White House and three executive branch departments ignored House subpoenas demanding those documents about Ukraine, but news outlets and outside groups have used the public records law, the Freedom of Information Act , or FOIA, to push the issue into the courts and seek access to documents. Favorable moves from federal judges forced the administration to start releasing the files more quickly than usual.

"The Freedom of Information Act has been an incredibly important safety valve against the administration's blockade of documents," said Austin Evers, executive director of American Oversight, one of the liberal-leaning transparency groups that sued to get Ukraine documents.

Despite these early successes, FOIA is an imperfect tool for the public to gain access to government documents.

Federal agencies can be slow in responding to initial document requests, often forcing lawsuits, which add to the delays. These lawsuits by news organizations have slowly uncovered snippets of new information, but the Trump administration has used its power to hold back much of the material using legal exemptions to FOIA requests.

For instance, on Friday the Trump administration withheld from The New York Times 20 emails about Ukraine between a top aide to President Donald Trump's acting chief of staff and an official at the White House Office of Management and Budget, citing executive privilege.

Major redactions -- omitting entire emails or blacking-out full pages -- are regularly used by the government to keep information secret. In response to many FOIA requests, federal agencies sometimes won't even confirm or deny the existence of records, unless forced by a judge.

But the latest revelations about the holdup of Ukraine aid highlighted the benefits -- and dangers -- of Trump's blockade. On one hand, it stopped Democrats from acquiring more damaging material against Trump. At the same time, it was always unlikely that these files would stay secret forever.

The fact that these documents are being released or can be in the future could play into how a Senate trial plays out. Democrats believe the new documents could boost public pressure for a more expansive trial. And Republican senators are being placed in the awkward position of acquitting Trump now, only to see damning emails emerge in the future.

There was another bombshell Monday when former national security adviser John Bolton said he would testify at the Senate trial if he is subpoenaed. Taken together, these developments speak to the fluid situation unfolding in Washington as the contours of Trump's trial take shape.

A flood of FOIA requests

Dozens of organizations sued for documents under FOIA after a whistleblower complaint came out in September, which alleged that Trump abused his powers by pressuring Ukrainian President Volodymyr Zelensky to help his re-election campaign, claims that formed the core of the impeachment case against Trump.

So, the onus fell back onto the FOIA cases that were brought by a wide array of organizations. These groups include news organizations, the liberal-leaning American Oversight, the nonpartisan Center for Public Integrity and Judicial Watch, a conservative group.

Some of the groups asked courts to look at their cases immediately, asking for emergency intervention. Federal judges in the DC District Court, who typically handle hundreds of FOIA lawsuits, held hearings and set plans for many of the documents to publicly released. That allowed the groups to negotiate directly with the agencies -- and gave them some leverage, too.

"Only an informed electorate can develop its opinions and persuasively petition its elected officials to act in ways which further the aims of those opinions," federal Judge Colleen Kollar-Kotelly wrote in late November, ordering the Pentagon and OMB to process records requests about Ukraine military aid before the end of the year.

With impeachment looming and federal judges taking notice, agencies began cutting deals with requesters who had sued them to make some of the records public. That's how The Times got its own response from OMB . The Times previously reported on other emails , described by sources, about the role of acting White House chief of staff Mick Mulvaney and his allies at the OMB. Some of the officials named in the emails defied House subpoenas to give depositions behind closed doors.

The Center for Public Integrity obtained some documents under Kollar-Kotelly's order at the end of December, with heavy redactions. The sections that were not blacked-out revealed for the first time that political appointees at OMB ordered the Pentagon to freeze aide for Ukraine about 90 minutes after Trump's controversial July 25 call with Zelensky.

The national security website Just Security learned more about what was under the redactions through sources who provided the outlet with parts of some emails that hadn't been seen before. The emails indicated that Pentagon officials were worried that the freeze might be illegal, but Trump appointees at OMB pushed them to keep the hold in place because of a "clear direction from (Trump)."

None of these lawsuits are close to over, and could drag out for months to come, stretching beyond Trump's trial in the Senate, which many Republicans hope will be wrapped up quickly.

Sen. Marco Rubio, a Florida Republican, said Monday that the trial should be "constrained" to the evidence the House had when it voted to impeach Trump. This drew a rebuke from Sen. Brian Schatz, a Hawaii Democrat, who said this guaranteed that the Senate could not hold a "fair trial." (They were reacting to Bolton's announcement that he would testify under subpoena.)

Trump has always maintained that he did nothing wrong with Ukraine and never linked US military assistance, or other official actions, to the announcement of an investigation into Biden.

More emails are on the way

American Oversight expects to receive two more sets of Ukraine-related documents this week.

The first is expected Wednesday from the State Department, which says it will produce emails about Trump attorney Rudy Giuliani and other private citizens who dealt with Trump on Ukraine issues. Giuliani played a leading role in the effort to pressure the Ukrainian government to announce the investigations into Biden.

On Friday, American Oversight is expecting documents about calendars and other records about Kurt Volker, the former US special envoy for Ukraine. A Trump appointee who quit after the Ukraine scandal broke , Volker was a key middleman between the two countries, and connected Giuliani with a senior adviser to Zelensky, the Ukrainian president.

These documents are likely to have heavy redactions like the other Ukraine-related files recently released under FOIA. But they could shape the public's understanding of how Trump relied on Giuliani to circumvent and undermine the State Department. Volker was also one of the "three amigos" who were plugged into this effort, which was kept away from many career officials.

Without these public records lawsuits, and the reporting from these news organizations, many of these documents would likely still be secret -- kept under wraps by the Trump administration.

Some liberal commentators have questioned the legitimacy of these redactions, and others have even accused the Trump Justice Department of participating in an intentional cover-up.

Many public records requests related to the Ukraine and Trump's impeachment, including dozens of requests filed last year by CNN, have yet to see final responses from the government.