UPDATE: After initially reporting Strzok make the alteration, the Washington Post edited a story overnight attributing the change to a “low level lawyer.” From Fox News:

The new evidence concerning the altered document, which pertained to the FBI's FISA court warrant application to surveil Page, is expected to be outlined in Horowitz's upcoming report. CNN first reported the news, which was largely confirmed by The Washington Post. The Post, hours after publishing its story, conspicuously removed the portion of its reporting that the FBI employee involved was underneath Peter Strzok, the FBI's since-fired head of counterintelligence. The Post did not offer an explanation for the change, which occurred shortly after midnight.

***Original post***

CNN legal analyst and former U.S. Attorney for the Southern District of New York Preet Bharara, who was fired by President Trump in 2017, says the news that former FBI agent Peter Strzok allegedly altered or forged a FISA warrant application used against Trump campaign associate Carter Page is "alarming."

"That's kind of an alarming bit of news," Bharara said. "If there was an FBI agent sworn to uphold the constitution who can be proven to have altered a document in connection with a legal proceeding, including the obtaining of a FISA warrant, that's really serious. It doesn't get more serious on that."

"That's not a good thing. It's a terrible thing," he continued. "Law enforcement agents and prosecutors who work with them are sworn to uphold the constitution, sure, but actually even exceed the protections of the constitution. They have to be of the utmost integrity and the utmost candor. Especially when you're making a representation to the court, which is what a FISA application is. It's gotta be on the up-and-up."

Breaking on @CNN: "This is exactly what people close to @realDonaldTrump have been saying, the FBI committed wrongdoing starting this investigation."@PreetBharara says altering a document to obtain a FISA warrant is "really serious, it doesn’t get a lot more serious than that." pic.twitter.com/IlzUlpwxdI — Robby Starbuck (@robbystarbuck) November 22, 2019

Keep in mind Strzok wasn't just an agent, he was a senior counterintelligence agent in the Russia investigation and during the FBI's investigation into Hillary Clinton. He was fired in August 2018 after text messages about an "insurance policy" if Trump won and other biases were discovered by the Department of Justice Inspector General.