A committee of the US House of Representatives is seeking documents alleging obstruction of justice, corruption and abuse of power by President Donald Trump and his aides.
Judiciary committee chairman Jerrold Nadler told ABC news that 60 individuals and entities would be receiving requests from Monday.
Mr Nadler said he believed Mr Trump had obstructed justice.
But any impeachment move would depend on the results of the inquiry.
President Trump has consistently denied any wrongdoing and accused Democrats of a witch hunt.
On Saturday, he launched a furious attack on Special Counsel Robert Mueller, railing against the inquiry he is leading into alleged collusion between his campaign and Russia.
Mr Mueller is expected to hand in his report to the attorney general shortly.
Targeting Mr Mueller repeatedly – as well as firing the former FBI chief over the Russia inquiry – were among the issues Mr Nadler cited as “clear” cases of obstruction of justice by President Trump.
But the Democratic congressman said “we do not have the evidence” to start an impeachment procedure against the president.
“Impeachment is a long way down the road, we don’t have the facts yet, but we’re going to initiate proper investigations,” Mr Nadler told This Week.
“Tomorrow [Monday], we will be issuing document requests to over 60 different people and individuals from the White House to the Department of Justice,” he said.
Among those receiving the requests would be Donald Trump Junior.
Democrats are now in control of the House of Representatives, with Republicans holding the Senate.