As families prepare to bury victims of another US mass shooting, grief mixed with anger amid signs of lapses in school security and indications law enforcement may have missed clues about the gunman's plans.

One distraught mother who said she had just spent two hours making funeral preparations for her 14-year-old child expressed disbelief a gunman could stroll into school and open fire, and she appealed to President Donald Trump to take action.

Nikolas Cruz, 19, identified as a former student at Marjory Stoneman Douglas High School in Parkland, Florida, walked into the school on Wednesday and opened fire with an assault rifle, killing 17 students and facility members and injuring 15 others, police said.

He had been expelled from the school for disciplinary reasons.

The shooting has raised questions among parents about the adequacy of school security measures and renewed national debate about the epidemic of gun violence in American schools.

"How do we allow a gunman to come into our children's school? How did they get through security? What security is there?" Lori Alhadeff shouted into the camera in an emotionally raw appearance on CNN.

"The gunman, the crazy person, just walks right into the school, knocks down the window to my child's door and starts shooting, shooting her ...," cried Alhadeff, whose daughter Alyssa was among the dead.

Cruz, charged with 17 counts of premeditated murder, made a brief initial court appearance on Thursday, in which he was ordered held without bond.

"He's a broken human being," his lawyer, public defender Melissa McNeill, told reporters. "He's sad, he's mournful, he's remorseful."

Cruz may have foreshadowed the attack in a comment on YouTube, with the FBI disclosing it received a tip in September about the message that read: "I'm going to be a professional school shooter," by a user named Nikolas Cruz.

However, FBI agents had no information pointing to the "time, location or true identity" of the person behind the message, Robert Lasky, special agent in charge of the FBI's Jacksonville office, told reporters.

After the shooting Cruz walked out of the school and was later spotted and detained by a police officer in the adjacent town of Coconut Creek.

Former classmates have described Cruz as a social outcast with a reputation as a troublemaker, as well as someone who was "crazy about guns."

"It is not enough to simply take actions that make us feel like we are making a difference," Trump said at the White House in a speech that emphasised school safety and mental health while avoiding any mention of gun policy. "We must actually make that difference."

Democrats in the US House of Representatives criticised the Republican leadership for refusing to take up legislation on tightening background checks for prospective gun buyers.

Some gun control proponents and legal experts said Wednesday's shooting might have been averted if Florida had laws allowing police and family members to obtain restraining orders barring people suspected of being a threat from possessing guns.