"We don't really know what took place here, and we need to find out," Schiff, D-Calif., told ABC News chief anchor George Stephanopoulos in an interview today on "Good Morning America."
"We need to get those notes, if they exist, that Director Comey took of this conversation," Schiff added.
A source close to Comey confirmed to ABC News that the former FBI director, who was fired last week, was asked by Trump in February to end the investigation into Flynn's actions, according to a memo Comey wrote about his conversation with the president.
In the memo, which was shared with top FBI associates, Comey wrote that Trump said, "I hope you can let this go," in relation to the probe.
"I hope you can see your way clear to letting this go, to letting Flynn go," Trump said to Comey, according to the source who read the memo. "He is a good guy. I hope you can let this go."
The New York Times was the first on Tuesday to report the discussion and subsequent memo.
ABC News has not seen the memo, and Comey has not commented on the matter.
The White House has denied that Trump tried to block the probe, saying in a statement, "While the president has repeatedly expressed his view that Gen. Flynn is a decent man who served and protected our country, the president has never asked Mr. Comey or anyone else to end any investigation, including any investigation involving Gen. Flynn."
"The president has the utmost respect for our law enforcement agencies and all investigations," the statement continues. "This is not a truthful or accurate portrayal of the conversation between the president and Mr. Comey."
On "GMA," Schiff said Congress is prepared to issue a subpoena to obtain Comey's memo and launch an investigation if necessary.
"Congress will need to subpoena them if indeed they're not provided voluntarily. I certainly support that. I think there are many committees that will," Schiff said. "Congress really needs to get to the bottom of this."
"Those notes could be very powerful evidence," he added.
ABC News' Jack Date, Adam Kelsey, Kelly McCarthy, Benjamin Siegel, Pierre Thomas and Cecilia Vega contributed to this report.