In the day's other headlines: Russian officials insisted again that Ukraine played a role in Friday's concert hall attack outside Moscow.

A branch of the Islamic State group has claimed responsibility for killing 139 people there.

But as workers spent another day clearing debris, the head of Russia's top security agency accused Kyiv and its allies.

ALEXANDER BORTNIKOV, Director, Federal Security Service (through translator): We believe that the action was prepared by the Islamist radicals and facilitated by Western special services.

As for the Ukrainian intelligence services, I think they are directly involved too.

WILLIAM BRANGHAM: The Russians have not presented evidence to support their claim, and Ukraine has denied any role.

A Moscow court has extended the detention of American Evan Gershkovich again, this time through June.

The Wall Street Journal reporter already spent a year behind bars, accused of espionage.

After he appeared in court today, the U.S. ambassador to Russia again rejected the charges and said the case is purely political.

LYNNE TRACY, U.S.

Ambassador to Russia: As we cross the one-year mark, the Russian government has yet to present any evidence to substantiate it's accusations.

Evan's case is not about evidence, due process, or rule of law.

It is about using American citizens as pawns to achieve political ends.

WILLIAM BRANGHAM: Gershkovich and The Journal have denied the espionage allegations as well.

A British court today blocked WikiLeaks founder Julian Assange from being extradited to the U.S., for now.

The U.S. wants to prosecute Assange for publication of classified American documents in 2010.

The court said that the U.S. must guarantee Assange is afforded the same First Amendment protections as a United States citizen, and that the death penalty is not imposed.

Washington has three weeks to give that guarantee, or a new appeals process will begin.

In Venezuela, the main opposition coalition says it is undeterred after its presidential candidate was barred overnight from the upcoming election.

Corina Yoris, an 80-year-old newcomer, had been chosen to face authoritarian president Nicolas Maduro in July.

The groups original candidate was also barred from the election.

Back in this country, a state judge in New York imposed a gag order on former President Trump for his hush money trial next month.

The judge's ruling said: "It is without question that the imminency of the risk of harm is now paramount."

The gag order bars public statements about witnesses, prosecutors, court staff, and jurors.

Independent presidential candidate Robert F. Kennedy Jr. announced Nicole Shanahan as his running mate today.

Shanahan is 38, a lawyer and philanthropist from California, and has never held elected office.

But in Oakland, California, Kennedy told supporters that Shanahan met all his requirements for vice president.

ROBERT F. KENNEDY JR., Presidential Candidate: I wanted a partner who is a gifted administrator, but also possesses the gift of curiosity, an open, inquiring mind, and the confidence to change even her strongest opinions in the face of contrary evidence.

WILLIAM BRANGHAM: Kennedy chose a running mate early because some states require it before he applies for ballot access.

Texas Attorney General Ken Paxton will pay nearly $300,000 to settle a criminal securities fraud case.

The high-profile Republican faced trial next month on charges of duping investors in a tech start-up.

Under today's agreement, Paxton makes restitution and does community service, but keeps his elected position.

Visa and Mastercard have reached a roughly $30 billion settlement with merchants over so-called swipe fees.

The companies charge per transaction, and that cost is usually passed on to the consumer.

The settlement will cap those fees through 2030, so shoppers could save billions of dollars.

And on Wall Street: Stocks edged lower in lackluster trading.

The Dow Jones industrial average lost 31 points to close at 39282.

The Nasdaq fell 68 points.

The S&P 500 slipped 14.

Still to come on the "NewsHour": a troubled rollout causes more delays for students seeking financial aid; an acclaimed foreign correspondent discusses his memoir and his battle with terminal cancer; the changing landscape of podcasts; plus much more.