Transcript for What's next for North Korea, U.S. relations: Part 6

Reporter: Otto's now all too familiar perp walk was reprised only a month later, in the same courtroom by another American. That's businessman Kim dong-chul, one of the three Americans still detained in North Korea. The other two are college professors Tony Kim and Kim hak-song. We still have three Americans left there that need to come home, and we need to get them out. Reporter: Former governor bill Richardson, who has negotiated with the regime in the past, has a stern warning to the estimated 1,000 Americans traveling to North Korea every year. Don't go to North Korea. It's too dangerous. And you may end up like Otto Warmbier. Unjustly killed. This is a human being detained, 21 years old with a great family a grieving family. North Korea should not get away with this. Reporter: Could an international incident like the death of Otto ignite a larger conflict? Do you think there's any chance of a U.S. Military attack? Over 10,000 artillery pieces is 60 seconds away from Seoul. We have some 20 million people living there. We have some 200,000 American expats living there. We have 28,500 American soldiers living there. You -- so, just forget about the nuclear weapon, just conventi conventional artillery alone can devastate Seoul. So, this is not really an option that we can pursue. Reporter: When I visited North Korea earlier this year, the vice foreign minister gave me a chilling hint of the regime's state of mind. There's this old Korean saying that if somebody points a gun at you, you'll point the gun back, but if we offer you a rice cake, you'll offer a rice cake back. Do you think in this condition right now, do you think -- are we going to have peace with a cake or are we going to have a bullet? Translator: The exact expression is that if America pulls out a knife, we will pull out a sword and if America puts out a gun, we will pull out a cannon. Reporter: So, what other options are on the table? The U.S. Will likely keep up its reported cyber attacks on North Korea's missile system, while continuing to push for China's cooperation in the region. China accounts for roughly 90% of North Korea's trade. China is our ally in an effort to quell North Korea, but they still haven't done enough. Reporter: More likely, the next step would be economic sanctions against Chinese banks that do business with north Korea. Think of the north Korean regime as a large mafia family. They make their money off of everything illicit. Drugs, cigarettes, money laundering, arms smuggling. The U.S. Will go in and try to stop those, cut those off and essentially cut the life blood of the north Korean economy off. Reporter: And as America mourns Otto's death -- I wish there was a way to know that you are in the good old days before you actually left them. Reporter: His friends are remembering his bright spirit. He said, we'll always have each other, talking about our class, and I think now more than ever that's really -- really showing to be true, and Otto would want that. And we will have the reruns, the memories we created to be played over and over again. Thank you.

This transcript has been automatically generated and may not be 100% accurate.