Amid international concerns about security, the Sochi Games completed the first day of contests Thursday with high excitement and -- equally important -- no major adverse incident.

After all the anxiety about a terror strike, controversy over gay rights and ridicule over poor preparations, the Winter Olympics commenced smoothly enough, as qualification events were held in the men's and women's slopestyle, women's moguls and team figure skating.

The last of the athletes arrived at the Russian venue on Thursday, including the bulk of Team USA, and the Olympians will hold the Games' official opening ceremony Friday.

On the eve of that ceremony, Dmitry Chernyshenko, head of the Games, promised Sochi will be "the safest place on Earth during the Olympics."

Thursday's developments involved more than just sports.

SECURITY

There's no doubt it's the issue at the forefront of people's minds.

Russia has drafted some 37,000 police and security officers to handle security in Sochi. But that's not been enough to assuage everyone's fears.

Toothpaste terror: A day after the United States warned of how explosive materials could be concealed in toothpaste or cosmetic tubes, its government Thursday temporarily banned all liquids, gels, aerosols and powders in carry-on luggage on flights between the United States and Russia.

A law enforcement source told CNN the effort is intended as a very targeted response to the threat that became public Wednesday, and should have a minimal impact on the traveling public. The Transportation Security Administration ordered airlines to ban the items from carry-on bags, but allow them in checked bags.

U.S. partnership: Meanwhile, U.S. authorities are working with the Russians and other countries to try to disrupt several possible threats, including the toothpaste tube concern, a U.S. intelligence source said Thursday.

The threats vary in credibility, and the biggest one traces to the group Imarat Kavkaz in Russia, which has publicly said its followers will try to disrupt the games, the official said.

Meanwhile, Sen. Dianne Feinstein, D-California, told CNN she's read the threat reports about the Olympics. "The threat stream is credible, I think it's real," she said.

Feinstein noted at least one terrorist group -- the Caucasus Emirate -- "is conceivably capable of carrying out an attack," she said.

"If I had a son or daughter performing in the Olympics, I would go. Now that I don't, I probably would not," said Feinstein. "I've seen the threat stream, and I know the opposition, and the opposition is dogged. It is continuing, it is threatening, and hopefully arrests can be made and any attack can be stopped."

Private protection: The U.S. ski and snowboarding team has hired a private security firm, Global Rescue, to provide protection. It's not clear how much the firm could do in the event of a major incident, when Russian forces will be in charge, but it has been gathering intelligence on the ground and will provide an extra layer of protection as athletes travel around.

Ships for safety: Meanwhile, two U.S. Navy ships have steamed into the Black Sea, where they will be ready to help if any mass evacuation of U.S. citizens is needed. U.S. security officials have also been working with their Russian counterparts on how to keep the Games safe against the backdrop of a regional separatist movement that has used terrorism in the past and has threatened to use it during Sochi's Olympic Games.

Targets of threats: Americans are not the only ones who are jittery. Austria said this week that two of its female athletes had been the target of specific threats. Austrian media reported an anonymous letter was sent warning Alpine skier Bernadette Schild and skeleton racer Janine Flock they could be kidnapped.

We've heard it before: It's not the first time security issues have dominated the build-up to the Olympics -- Britain parked missile batteries on apartment block roofs and a warship on the River Thames before the 2012 Games. The Salt Lake City Winter Olympics in 2002 were held amid heightened security only months after the 9/11 terror attacks in the United States -- and the 1996 Summer Games in Atlanta were subject to a terror attack.

Being an athlete: Upon arriving to Sochi, U.S. ice skater Maia Shibutani didn't hear of the explosive toothpaste threat. She and brother Alex, her skating partner, had been too busy traveling.

But she can't be distracted by the threats, as serious as they are, she said.

"Yesterday we were doing processing, and then we were traveling all morning, so, no, we hadn't heard about that," she said Thursday of the toothpaste threats. "But I think that, really our job here is to just focus on what we can control and what we can do, and that's how we're going to perform at our first Olympics."

PREPARATIONS

When Russia bid to host its first Winter Olympics in 2007, a document quoted an expected cost of around $12 billion. That figure has ballooned to around $50 billion. That's more than four times over budget and surpasses Beijing's 2008 Summer Games -- making it the most expensive Olympics ever, summer or winter.

Russia had less than seven years to transform what was a fairly low-key seaside resort town into a Winter Olympics venue -- a project that required staggering feats of engineering in building a new freeway and rail link up a mountain, and a ski resort on the top. And yet questions over Sochi's readiness have dogged the final run-up to the Games.