As Congress returns from recess, US House Democrats plan to ramp up their impeachment probe into President Donald Trump.

The Democratic-led US House of Representatives will ramp up its impeachment inquiry into President Donald Trump this week as Congress returns after a two-week recess, with closed-door testimony from current and former administration officials looming.

The impeachment inquiry focuses on a July 25 phone call in which Trump pressed Ukrainian President Volodymyr Zelensky to investigate former Vice President Joe Biden, a top contender for the Democratic presidential nomination, and his businessman son, Hunter.

Democrats have accused Trump of pressuring Ukraine to find incriminating information on Biden after withholding nearly $400m in US security aid to Ukraine. There has been no evidence of wrongdoing by the Bidens.

Trump maintains he has done nothing wrong and has called the impeachment inquiry a “witch-hunt”.

This week’s headline event will likely be on Thursday, with the scheduled deposition of Gordon Sondland, the US ambassador to the European Union. He participated in a text message exchanged that Democrats have said reveals the internal concern among aides that the Republican president’s pressure on Ukraine to investigate Biden was improper.

As House Democrats make their way back to Washington, DC, here is a look at this week’s scheduled testimony and deadline.

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Monday, October 14

Testimony: Fiona Hall, former Russia adviser

Fiona Hall, who served on Trump’s National Security Council as the senior director for European and Russian affairs, is scheduled to give a closed-door deposition to House investigators on Monday.

Testimony: Semyon Kislin, associate of Trump’s lawyer, Giuliani

Semyon “Sam” Kislin, an associate of Trump’s lawyer, Rudy Giuliani, is scheduled to testify before House investigators in a closed-door session on Monday.

Kislin also has business ties to Trump and served on Giuliani’s city economic development corporation when the lawyer was mayor of New York City.

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It is unclear whether he will attend the deposition on Monday.

Read more from Al Jazeera’s Investigative Unit about Kislin here.

Deadline: Sondland documents

The House Intelligence Committee subpoenaed Ambassador Sondland to provide documents on Monday related to the Trump administration’s dealings with Ukraine.

“Ambassador Sondland’s testimony and documents are vital, and that is precisely why the administration is now blocking his testimony and withholding his documents,” three Democrat committee chairs said in a joint statement.

The White House previously blocked Sondland from testifying. He is now scheduled to be deposed on Thursday, but his lawyers said he would not be authorised to release documents.

Tuesday, October 15

Deadline: Pence documents

House Democrats asked Vice President Mike Pence to turn over documents related to a meeting he held with Zelensky and a separate call between Trump and the Ukrainian leader.

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The Democratic chairmen of the three House committees leading the investigation gave Pence until Tuesday to produce the records.

The White House has said it would not cooperate with the impeachment inquiry.

Subpoena deadline: Giuliani document

The House Intelligence Committee issued a subpoena to Giuliani earlier this month, setting an October 15 deadline for him to turn over documents related to his efforts to work with the Ukrainians to investigate Biden.

The former New York mayor said he would not comply with the subpoena, saying it raised issues of “attorney client and other privileges”.

The committee warned that failure to comply could amount to obstruction or be used as an adverse inference against Trump and Giuliani.

Subpoena deadline: Pentagon documents

Last week, congressional Democrats issued subpoenas to the Pentagon, seeking documents related to Trump’s decision to withhold military assistance from Ukraine. They set an October 15 deadline.

The Trump administration has said it would not cooperate with the impeachment inquiry.

Subpoena deadline: White House Budget Office documents

House Democrats also gave an October 15 deadline for the White House Budget Office to turn over documents related to the delay of US military assistance to Ukraine.

The Trump administration has said it would not cooperate with the impeachment inquiry.

Testimony: George Kent, deputy assistant secretary of state

George Kent, the US deputy assistant secretary of state in the European and Eurasian Bureau, is scheduled to testify on Tuesday.

Kent was blocked by the Trump administration from testifying at his first scheduled deposition but has since indicated he will appear in a closed-door session.

Wednesday, October 16

Subpoena deadline: Lev Parnas and Igor Fruman documents

Last week, Lev Parnas and Igor Fruman, business associates of Giuliani, were arrested as they attempted to leave the United States.

They were charged with making illegal donations to US politicians, including a group supporting Trump.

US President Trump’s personal lawyer Rudy Giuliani and his associates were heavily involved in dealings with Ukraine [File: Aram Roston/Reuters]

Separately, the House Intelligence Committee subpoenaed the two men for documents as part of the impeachment inquiry.

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The two men allegedly sought the removal of US Ambassador to Ukraine Marie Yovanovitch, who was recalled by Trump in May. Yovanovitch testified last week that Trump pushed for her removal.

Parnas and Fruman had previously indicated they would not cooperate with the impeachment investigation.

Thursday, October 17

Testimony: Gordon Sondland, US ambassador to the EU

Gordon Sondland, the US ambassador to the EU, is scheduled to testify on Thursday.

Sondland, a political appointee and Trump political donor rather than a career diplomat, is expected to discuss the text message exchange with Bill Taylor, the top US diplomat in Ukraine.

Trump is joined by Sondland as he arrives at Melsbroek Air Base, in Brussels, Belgium [File: Pablo Martinez Monsivais/AP Photo]

Sondland is expected to be asked why he relayed from Trump to other diplomats that the president said no “quid pro quos” connecting the Biden investigation with the US aid. Quid pro quo is a Latin term meaning a favour for a favour.

Friday, October 18

Subpoena deadline: Rick Perry documents

US Energy Secretary Rick Perry was subpoenaed by the House Intelligence Committee to produce documents related to his alleged role in the Trump administration’s dealings with Ukraine.

Perry was sent by Trump instead of Vice President Pence to attend Ukraine President Zelensky’s inauguration in Kiev in May.

Trump has said Perry was the one who had encouraged him to take the call with the Ukrainian leader. A spokeswoman for Perry said the energy secretary had wanted Trump to speak to Zelensky about matters related to energy.

Subpoena deadline: White House documents

The House Oversight Committee subpoenaed White House Chief of Staff Mick Mulvaney for key documents related to the impeachment inquiry.

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The subpoena came after the White House repeatedly denied to turn over requested documents.

“We deeply regret that President Trump has put us – and the nation – in this position, but his actions have left us with no choice but to issue this subpoena,” the chairmen of three House committees wrote to Mulvaney.