Gunman Kills 3 Marines in Afghanistan

Shooter dressed in Afghan uniform attacks U.S. Marines training soldiers.
3:00 | 08/10/12

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Transcript for Gunman Kills 3 Marines in Afghanistan
We move to a stark reminder tonight that tens of thousands of american troops are still very much in harm's way in afghanistan. A gunman in an afghan uniform killing three u.S. Troops, just the latest in an alarming series of attacks. Here tonight, abc's martha raddatz now. Reporter: The attacks have been unrelenting. From this recent strike, watch that massive truck bomb drive to the entrance of a remote u.S. Base -- injuring more than 100. To today's murderous hit that left three special operations forces dead. The americans had likely been trying to help train the very man, wearing an afghan security uniform, who would gun them down. This young afghan solider was welcomed as a hero after recently defecting to the taliban. And it seems there are a growing number just like him. So far this year, attacks by afghan forces have left at least 31 coalition troops dead. A number that could far exceed last year's total. It definitely does start to effect the trust of both sides working together. Reporter: Today's attack following a heartbreaking blow earlier this week, a suicide attack that killed three senior u.S. Military leaders whose remains were returned to dover air base late today. The dead include command sergeant major kevin griffin, 38-year-old walter gray, an air force major and major thomas kennedy, a west point graduate with a twin boy and girl at home. A usaid worker was killed in the attack, as well. Loss of any soldier is a tragedy. But when you lose a senior individual like that, it has a chilling effect on the unit. Reporter: The command sergeant major, in particular, looks out for the well being of his soldiers, is a real caretaker. He will be replaced by someone who is no doubt, capable, but it is like trying to replace a parent, david. It just won't be the same. All right, martha raddatz in our washington bureau. Martha, thank you.

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

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