Federal agents say they that, based on collected evidence, they cannot yet rule out the possibility that Nicolas Ivie died as a result of "friendly fire," (or an accidental shooting from fellow officers) on Tuesday morning, Oct. 2.

NBCnews.com has this brief:

Federal investigators have told NBC News they are examining whether the shootings of Border Patrol agents early Tuesday morning were the result of friendly fire — officers accidentally shooting each other.

This comes not long after the announcement that Mexican officials have detained and questioned two people in connection with Ivie's death.

From the Washington Post:

The official, who spoke on condition of anonymity because he was not authorized to release the information, said it was unclear if there was strong evidence linking the men to the shooting of Agent Nicholas Ivie.

...

Brenda Nath, an FBI spokeswoman in Arizona, and Border Patrol officials in Arizona declined to comment on the detention of the two men in Mexico. The Cochise County sheriff’s office, which is also investigating the shooting, didn’t immediately respond to requests for comment.

Lydia Antonio, a spokeswoman for the Mexican Embassy in Washington, confirmed the two detentions, but declined to say what prompted them and what made authorities suspect the two might be involved in the shooting.

The speculation of friendly fire is just that — speculation, and nothing more. We'll have more to you as we hear about it.