Students, parents, educators and politicians were among the sad and angry voices Thursday calling for gun reform in the United States, where violent mass shootings are colliding with constitutional rights "to keep and bear arms."

The hashtag #GunReformNow remained the top trending topic on social media, where many called for a way to curb the sale and use of high-powered assault rifles that typically are used in mass shootings in the U.S.

The Valentine's Day shooting at Marjory Stoneman Douglas High School in Broward County, Florida, in which 17 people were killed and at least 15 were injured, was the 18th mass shooting of the new year, according to Everytown for Gun Safety, a nonprofit organization in the U.S.

Video footage showed high school students crouched and sprawled on a classroom floor, crying and screaming as the sound of gunshots are heard in the background. Students who had fled to safety Wednesday lauded teachers who shielded them as the gunman made his way through the school.

Arrested was Nikolas Cruz, 19, a former student. He was charged with 17 counts of premeditated murder.

Speaking on Capitol Hill, Democratic Senator Bill Nelson from Florida implored his colleagues to address the issue.

"Enough is enough," said Nelson, after a minute of silence to recognize the tragedy. "At some point as a society, we have got to come together and put a stop to this ... I have hunted all my life, I've had guns all my life, I still hunt with my son, but an AR-15 is not for hunting.

"It's for killing."

Nelson said he would "beg my colleagues to take commonsense actions that we all know will help protect our children and fellow citizens. … When is enough enough?" he concluded, shaking his head.

Others off- and online, pointed a finger at politicians who offer "thoughts and prayers" after shootings, but who accept large contributions from the powerful gun lobby, the National Rifle Association, or NRA. News outlets issued lists of lawmakers who have taken the most contributions from the NRA.

Republican Senators John McCain of Arizona, Marco Rubio of Florida and Roy Blunt of Missouri), among others, were cited.

Rubio tweeted that he and his wife "were devastated and saddened by today's inexplicable tragedy …We join millions of Americans in praying for the victims, their families and all the students and teachers impacted by today's events."

Some Twitter users questioned whether the NRA contributions dissuaded the lawmakers from confronting the gun violence problem.

The NRA declined to comment to VOA about the Florida shooting, but the homepage on its website states that recently, NRA Chief Executive Officer Wayne LaPierre "let [gun-control politicians] and their anti-gun minions know that the NRA will not tolerate their use of a tragedy in their war against the Second Amendment."

The amendment to the U.S. Constitution and the Bill of Rights, written in 1791, says, "A well regulated militia, being necessary to the security of a free state, the right of the people to keep and bear arms, shall not be infringed."

Self-described conservative DrConsrvaMom argued on Twitter that guns should have more of a presence in schools for protection. On Twitter, she posted a graphic listing the places where guns are used for protection, from safety for the president to sporting events to banks.

"We defend our children with a sign that reads: This is a gun-free zone. And then call someone with a gun if there's an emergency," said DrConsrvaMom.

The teachers' union in the county where Marjory Stoneman Douglas High School is located released a statement.

"Our hearts are broken," Anna Fusco, president of the Broward Teachers Union, said in a statement Wednesday. "This is a day we will never forget, one on which we've lost precious lives in a senseless tragedy. It is impossible to make sense of the mass shooting at Marjory Stoneman Douglas High School today, yet somehow we will come together."