Good morning, NBC News readers.

Twenty years after Columbine ushered in the tragic era of school shootings, Colorado suffered another attack yesterday. A would-be Marine and his classmates disarmed one of the gunmen.

Here's what else we're watching today.

Iran steps away from parts of the nuclear deal

Iran has informed ambassadors from the United Kingdom, France, Germany, China and Russia that it would stop implementing parts of the landmark 2015 nuclear deal.

President Hassan Rouhani said in a speech broadcast on national television that the country would also resume high level enrichment of uranium if world powers did not keep their promises under the Obama-era agreement.

The announcement came on the anniversary of the U.S. withdrawal from what was originally a six-party deal and amid rising pressure from the Trump administration.

Secretary of State Mike Pompeo made an unannounced trip to Iraq on Tuesday, citing escalating tensions with Tehran.

'I started hearing screams'

One 18-year-old student was killed and eight others were injured in a shooting at a suburban Denver school.

Two student suspects are in custody, officials said.

The shooting at the STEM School Highlands Ranch comes less than a month after the area marked the 20th anniversary of the Columbine High School massacre.

The STEM campus is just eight miles southeast of Columbine, but after years of living with the threat of school shootings, students have been taught how to flee, hide and even fight back.

Brendan Bialy, a senior who plans to become a Marine after graduation, was one of a group of students who tackled and disarmed one of the two shooters Tuesday.

Before this incident, two people had been killed and six injured in three school shootings in 2019, according to NBC News research.

Meet Baby Sussex

Prince Harry and the former Meghan Markle, who is now the Duchess of Sussex, said they were caught up with the "magic" of parenthood as they offered the world their first glimpse of their newborn son Wednesday.

But they didn't reveal perhaps the most important detail about the latest royal baby: His name.

"It's magic, it's pretty amazing. I have the two best guys in the world so I'm really happy," the new mother said as Prince Harry cradled their son.

Democrats' constitutional clash with Trump ramps up

President Donald Trump and Congress wheeled closer to a constitutional crisis Tuesday, writes NBC News' Jonathan Allen.

But House Democrats remain divided by competing impulses on whether to pursue impeachment or to follow a more politically cautious path.

House Speaker Nancy Pelosi, D-Calif., pointed to her party's dilemma in comments on Tuesday.

"Trump is goading us to impeach him," she said.

'I had to find answers for the family'

Terre Haute Police Chief Shawn Keen knew he would be taking a risk by using up the last human sample he had from a 47-year-old crime scene, but had a feeling the gamble would pay off.

This week, it did.

Using DNA testing, Keen solved the 1972 murder of Indiana college student Pamela Milam — a crime that happened two years before the police chief was born.

“I had to find answers for the family,” Keen said. “Forty-seven years later, it still matters to them.”

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Plus

Trump's businesses lost more than $1 billion from 1985 to 1994, according to a report by The New York Times.

according to a report by The New York Times. Count on using your feet to get around: Uber and Lyft drivers plan to strike today to protest low wages and job insecurity.

Uber and Lyft drivers plan to strike today to protest low wages and job insecurity. Sen. Kamala Harris has blown away her Democratic rivals in fundraising in communities of color.

THINK about it

President Trump's base may love his immigration rhetoric, but they won't like its economic effects, financial researcher Evan Horowitz writes in an opinion piece.

Science + Tech = MACH

Here's why NASA just destroyed a simulated New York City with a huge fake asteroid.

Quote of the day

"We chose a small school because we thought they’d be safer, because we thought that everybody would know everybody.”

— Nyki Giasolli, the mother of two survivors of the Colorado STEM school shooting.

One thoughtful thing

Bestselling author Mark Manson believes the most important thing you can do is to choose what to care about.

Everything else is just white noise. See what he has to say below.

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Thanks, Petra