Vince Cohen makes a point during an interview in his office. (Andre Chung/For The Washington Post)

Longtime D.C. defense lawyer Michele Roberts recalled handling a gun-possession case she thought would be an easy win. When she saw her opponent, a prosecutor fresh out of law school, she laughed and warned him that if he didn’t dismiss the case, she would “embarrass” him in court.

“I used to have young prosecutors for lunch,” Roberts teased.

But the moment Vincent H. Cohen Jr. began his opening statement in that case in the late 1990s, Roberts said, she knew he was no lightweight.

“He made it very clear that this was going to be a trial. He was such a handful,” Roberts, now executive director of the National Basketball Players Association, said in a telephone interview from her New York offices. “He tried the case like a veteran. I made it up in my mind this kid is not going to beat me. I knew I was up against a real lawyer.”

That kind of drive pushed Cohen, 44, through a career that took him into the private sector and then back to the D.C. federal prosecutor’s office, recently as second-in-command. Next week, he will become acting U.S. attorney in the District, filling in after the resignation of U.S. Attorney Ronald C. Machen.

His move comes during a challenging time for the largest U.S. attorney’s office in the nation.

The office has come under increased scrutiny during its four-year investigation of former mayor Vincent C. Gray (D). Prosecutors publicly accused Gray of having detailed knowledge of a “shadow campaign” to illegally funnel $660,000 into his 2010 get-out-the-vote campaign. Gray has called the allegations “lies.”

Machen and Cohen, longtime friends, have overseen the Gray investigation since it was launched. Several of Gray’s aides have been prosecuted, but there have been no charges against the former mayor or an announcement that the investigation has ended.

In recent weeks, the U.S. Attorney’s Office also has publicly sparred with police and city crime lab officials. Prosecutors have said they are reviewing more than 100 cases that may have been affected as a result of what his office called faulty DNA analysis by the District’s DNA lab. And Machen recently alerted defense lawyers that a “problem” in a computer system used by D.C. police may have inadvertently omitted evidentiary information from police reports in many cases, dating back to 2012.

Machen often referred to Cohen as the “best” first assistant U.S. attorney in the country and said he expects complete “continuity” when Cohen takes over.

“He knows this office inside and out, and a lot of the successes we’ve had, he has played a significant role,” Machen said in an interview. As the longest-serving chief federal prosecutor in nearly four decades, Machen plans to return to private practice.

Machen and Cohen were hired at the office in 1997 by Eric H. Holder Jr., then U.S. attorney for the District. Cohen has served as Machen’s No. 2 since President Obama selected Machen as the District’s top prosecutor in 2010.

Machen said Cohen played a key role in some of the office’s biggest cases, including the 2013 prosecution of former Chicago congressman Jesse Jackson Jr., who pleaded guilty to defrauding his reelection campaign of more than $750,000, as well as the ongoing investigation that uncovered the largest domestic bribery and bid-rigging scheme in the history of federal contracting cases.

“Over the past five years, it has been an honor to serve the community where I was born and raised, and I am committed to continuing that work for the people of the District of Columbia,” Cohen said.

For “Vo,” the nickname of Cohen’s youth given to him by one of his older sisters who couldn’t pronounce his name when she was a child, law was the family business. His father, Vincent H. Cohen Sr., became one of the District’s first African American lawyers to rise to partner level, just three years after joining the Hogan & Hartson law firm in 1969. When the elder Cohen died on Christmas 2011 at age 75, nearly 1,000 Washington-area politicians, judges, lawyers and business executives, including Gray, Holder and D.C. Del. Eleanor Holmes Norton gathered to pay tribute at the memorial service at the Washington Convention Center.

As a youth, Cohen attended D.C. public schools until he was accepted into Sidwell Friends School in Northwest Washington, long before presidents Clinton and Obama sent their children to the school. During his senior year, Cohen was elected president of the school’s black student union, which included a handful of minority students at the school. There, Cohen tried to bridge a coalition among other handfuls of African American students at the District’s elite private schools such as Georgetown Visitation Preparatory School and St. John’s College High School.

He went on to attend Syracuse University and in 1995 graduated from its law school. Both he and Machen are Omega Psi Phi fraternity brothers.

After six years with the U.S. Attorney’s Office, Cohen joined the defense side at Hogan & Hartson, the firm where his father worked. At Hogan, Cohen specialized in white-collar criminal cases and employment law.

Cohen worked with another Hogan & Hartson lawyer from the firm’s New York office in 2005 to help investigate the genocide in Rwanda. That other prosecutor was Loretta B. Lynch, Obama’s nominee for U.S. attorney general. During several trips to Tanzania and Rwanda for meetings with the International Criminal Tribunal for Rwanda, Lynch and Cohen spent days interviewing dozens of witnesses and survivors.

Danny Onorato, a defense lawyer who formerly served as a prosecutor alongside Cohen, called him “fierce, measured and fair as a prosecutor.” Onorato also worked with Cohen years later when Onorato co-founded his own firm, which Cohen later joined as a partner, before returning to the U.S. Attorney’s Office.

Until a full-time U.S attorney is selected to serve at least through the 2016 presidential election, Cohen will be busy.

Prosecutors are preparing for the upcoming trial of Libyan terrorism suspect Ahmed Abu Khattala. And the conviction of Ingmar Guandique, an illegal immigrant from El Salvador who was found guilty of the 2001 slaying of federal intern Chandra Levy, is back in D.C. Superior Court on appeal. And on Friday, charges were filed against a former FBI agent accused of stealing heroin from the bureau’s evidence room.

In a statement, Holder recalled hiring Cohen in 1997, saying since then he has “stood out as an exceptional prosecutor, a fair and forceful leader and a committed public servant.”

Roberts, who also spent time as a D.C. public defender, says she won that gun case many years ago in which she sparred against Cohen.

“Well, I wouldn’t have shared that story if I had lost,” she mused. But she also gained respect for Cohen.

“He’s the complete package,” she said.