On the November day he was arrested on federal corruption charges, then-Prince George’s county executive Jack B. Johnson stood on the steps of the U.S. District Courthouse in Greenbelt and proclaimed his innocence.

“I just can’t wait for the facts to come out,” said Johnson, a Democrat who was in the waning days of his second term. “I’m absolutely convinced I’ll be vindicated.”

Four months later, Johnson, 62, again vowed to fight allegations that he accepted and solicited bribes. “I don’t even recognize the person being charged,” he said.

But on Tuesday, Johnson admitted in court that he took more than $400,000 in bribes, pleading guilty to two felonies. Prosecutors said they will seek prison time at a sentencing scheduled for Sept. 15.

Why the dramatic reversal?

Legal experts said Johnson, who was Prince George’s top prosecutor before he became county executive, may have initially intended to fight the charges but was swayed by the evidence the government gathered during a years-long investigation. Federal agents had wiretapped several phones, including Johnson’s cell, listening in on thousands of conversations, court papers say.

And Tuesday, authorities and defense lawyers revealed that two developers had been cooperating with investigators and had admitted their roles in the corruption scheme in secret court proceedings. One developer had been working with the government since 2008.

“It’s a very common process,” said Robert C. Bonsib, a defense lawyer who had worked as a prosecutor. “An individual goes from a position of believing they have some wiggle room to confronting the reality of a strong government case.”

Bonsib said it was virtually certain that, before Johnson decided to plead guilty, he and his attorney, Billy Martin, listened to some of the most damaging wiretapped conversations together. Martin did not return a phone call.

“I am certain [Johnson] had no idea how much of his activity had been captured in a form of evidence he could not rebut,” Bonsib said. “His own words sunk his ship.”

In one wiretapped conversation recounted in court filings, Johnson talked to another county official about shaking down a developer for $500,000.

“Why don’t you and me go to his house together . . . so he can’t wiggle out of [expletive],” Johnson said, according to court papers.

Johnson told the official that they should “get five hundred together” and that he would keep the larger part of the split. “If I can get myself about three hundred, um, I’ll be in good shape.”

Jacob S. Frankel, a former federal prosecutor who is now a defense lawyer in Potomac, said well-known defendants charged with crimes often proclaim their innocence at the beginning.

“Often when the accused is high-profile, there is a long period of denial,” Frankel said. “On some level, most in such a position rose to that level of power because of their ‘fight first’ instinct. So the initial response is to fight. It can also take a varying amount of time for the reality and the consequences of going to trial to sink in.”

Defendants who are convicted after a trial in federal court face harsher sentences than those who plead guilty, legal experts said.

“A lot of high-powered defendants say they are innocent at first, and some are,” said Sharon Weidenfeld, a private defense investigator. “Sometimes, after they’ve seen the government’s evidence and conducted their own investigation, they conclude it’s in their interest to take a plea.”

Even with Johnson’s guilty plea, advisory federal sentencing guidelines suggest a sentence of 11 to 13 years in prison.

Frankel said Johnson might be hoping that his guilty plea will help his wife, Leslie Johnson, 59, who is charged with conspiracy to commit witness and evidence tampering. Leslie Johnson was elected to a seat on the County Council days before she and her husband were arrested.

Earlier this month, a hearing at which Leslie Johnson (D-Mitchellville) was expected to plead guilty was canceled two days before it was to occur.

Maryland U.S. Attorney Rod J. Rosenstein said Tuesday that Jack Johnson’s plea does not affect the case against his wife.