Now we head overseas to the boiling unrest in egypt, the giant arab nation and a linchpin for america in the middle east. This map shows you one reason why. A major life line for the world's oil... See More
Now we head overseas to the boiling unrest in egypt, the giant arab nation and a linchpin for america in the middle east. This map shows you one reason why. A major life line for the world's oil supply drives through the heart of that country which is on the drink tonight. Alex marquardt is there. Reporter: This is what america fears most, a massacre that could launch civil war. How it unfolded, caught on tape. This soldier on the roof, allegedly firing into the crowds of civilans. Here a protester fires at soldiers. In the end more than 50 were killed and hundreds wounded, almost all supporters of egypt's ousted president mohammed morsi, shot by the army. Both sides blame the other for starting it. 29-year-old mahmoud foud, shot in the leg tells ust the army opened fire as he and others prayed at dawn. It's a dangerous turn, and america knows it. A quarter of the region's arabs live in egypt. The fear is that if millions here give up on democracy, they could turn to extremism, even terror. Where is my vote? Where is his vote? Reporter: Then there's the oil. Those 2 million barrels passing through egypt's suez canal every day. Already the price of oil has jumped 7% in the past week. That could soon be felt at the pump. As the death toll rose today, morsi's group, the muslim brotherhood, told its members to rise up against the army. Tonight new presidential elections were announced six months from now, but the question is can egypt go that long without slipping into chaos. Alex marquardt. Abc news, cairo.
This transcript has been automatically generated and may not be 100% accurate.