The U.S. National Transportation Safety Board will be participating in the multiagency investigation into the Titan sub incident, an NTSB spokesperson told ABC News.

The U.S. agency joins the U.S. Coast Guard and the Transportation Safety Board of Canada (TSB), which deployed its members to St. John’s, Newfoundland and Labrador.

The TSB says it will conduct a "safety investigation regarding the circumstances of this operation conducted by the Canadian-flagged vessel Polar Prince."

-ABC News' Amanda Maile

White House press secretary Karine Jean-Pierre said the administration's "hearts go out to the families and the loved ones of those who lost their lives" on the Titan submersible.

"They have been through a tragic ordeal over the past few days and we are keeping them in our thoughts and prayers," she said.

She also thanked all parties who participated in search and rescue efforts this week.

"We would also like to thank the United States Coast Guard, our Canadian, British and French partners, and everyone who participated in the search and rescue efforts these past couple of days," Jean-Pierre said. "This whole effort has been a testament to the skill and also the professionalism that the men and women who serve our nation continue to demonstrate, continue to show each and every day."

-ABC News' Justin Gomez

When the U.S. Coast Guard decided to launch a search and rescue operation for the missing OceanGate submersible near the Titanic wreckage site, they did so without OceanGate footing the bill, according to officials.

As a matter of Homeland Security policy and the law, the Coast Guard never asks a private company or individual to reimburse the government for the costs associated with search and rescue missions, according to the Coast Guard.

"The Coast Guard, as a matter of both law and policy, does not seek to recover the costs associated with Search and Rescue from the recipients of those services,” according to a U.S. Coast Guard spokesperson. "As a matter of law, 46 USC 2110(a)(5) prohibits the Coast Guard from collecting fees for the conduct of SAR operations: The Secretary may not collect a fee or charge under this subsection for any search or rescue service."

-ABC News' Luke Barr

OceanGate co-founder Guillermo Söhnlein is coming to the defense of late CEO Stockton Rush, one of those killed in the implosion of Titan, after criticism from director James Cameron, and others, who said the sub's carbon-fiber hull was dangerous.

"In this kind of community, there are completely different opinions and views about how to do things, how to design submersibles, how to engineer them, build them, how to operate in the dives," Söhnlein told the U.K.'s Times Radio on Friday. "But one thing that’s true of me and the other experts, is none of us were involved in the design, engineering, building, testing or even diving of the subs. So it’s impossible for anyone to really speculate from the outside."

Söhnlein co-founded OceanGate in 2009 with Rush and led several dives in the early days of the company. He left OceanGate in 2013, when Rush took over as CEO, but maintained a friendship and spoke to Rush just weeks before the doomed expedition, he said on Facebook.

Cameron, who not only directed the Oscar-winning film "Titanic" but is a prolific ocean explorer, criticized the use of a carbon-fiber hull in the construction of Titan, saying it was only a matter of time before it cracked under the pressure from repeated dives. Rush, in the past, defended its use, saying it had a better strength-to-buoyancy ratio than titanium.

"I was involved in the early phases of the overall development program during our predecessor subs to Titan, and I know from firsthand experience that we were extremely committed to safety and risk mitigation was a key part of the company culture," Söhnlein told Times Radio Friday.

The U.S. Navy said it's sending to the search site experts and equipment "designed to provide reliable deep ocean lifting capacity for the recovery of large, bulky, and heavy undersea objects."

The equipment, which can lift up to 60,000 pounds, and the experts will arrive in Canada Tuesday night, the Navy said.

National Security Council spokesperson John Kirby said earlier Tuesday that the Navy was"on standby" to help with the search for the missing submersible, because the Navy has "some deep-water capabilities that the Coast Guard wouldn’t necessarily have."

President Joe Biden is "watching events closely," Kirby said, adding that Biden and the White House offer their thoughts "to the crew onboard, as well as to the -- what is no doubt -- worried family members back on shore."