Trump shooting updates: Shooter flew drone near site hours before shooting

The gunman who fired on former President Donald Trump on Saturday should never have been able to get a "direct line" of sight, Homeland Security Secretary Alejandro Mayorkas said.

"A direct line of sight like that to the former president should not occur," Mayorkas told ABC News' on "Good Morning America" on Monday. "That's why president Biden directed an independent review of the incident."

"We have to learn everything about the assailant ... and we are going to really study the event independently and make recommendations to the Secret Service," he said.

3:30

Homeland Security chief speaks out after attempted assassination of Trump

Sec. Alejandro Mayorkas discusses the state of national security after the assassination attempt on former President Donald Trump.

ABCNews.com

The U.S. Secret Service has added additional security to former President Donald Trump's detail ahead of the Republican National Convention, the agency’s director, Kimberly Cheatle, said in a statement on Monday.

“In addition to the additional security enhancements we provided former President Trump's detail in June, we have also implemented changes to his security detail since Saturday to ensure his continued protection for the convention and the remainder of the campaign,” Cheatle said.

The former president will have additional tactical officers, including SWAT, added to his detail, along with other updates, a law enforcement source told ABC News.

-ABC News’ Pierre Thomas

U.S. Secret Service Director Kimberly Cheatle said her agency welcomes the independent review announced by President Joe Biden.

“We understand the importance of the independent review announced by President Biden yesterday and will participate fully,” Cheatle said in a statement issued on Monday. “We will also work with the appropriate Congressional committees on any oversight action.”

Questions have been raised about how the suspect in the assassination attempt was able to climb on top of a roof near former President Donald Trump’s rally on Saturday.

“The Secret Service is working with all involved Federal, state and local agencies to understand what happened, how it happened, and how we can prevent an incident like this from ever taking place again,” Cheatle said.

-ABC News’ Pierre Thomas and Kevin Shalvey

Shares of Trump Media & Technology Group, a company majority owned by former President Donald Trump, rose about 50% in early trading.

The stock, which is traded under the DJT ticker, climbed to about $45 a share in pre-market trading on Monday, the first day the market will be open after the attempted assassination.

-ABC News’ Kevin Shalvey