I agree that it's probably not Cuba itself doing it though. My own suspects, based on nothing but wild speculation, is that this is Russia, rogue elements in Cuba, China, or the CIA testing experimental technology.
As for why I imagine the CIA is testing this, the CIA has experimented on unwitting American citizens and servicemen with drugs and poisons in the past[1], so it's not outside their ken to do such a thing.
Perhaps the CIA is testing their new stealth harassment/assassination/mind reading tool and wants to test it on realistic targets but also not risk exposing the device to a hostile party. With US intelligence agencies perpetrating and investigating attacks on realistic targets, it may make a good test environment.
I'm not saying I believe it is the CIA, obviously I have no evidence and this is just speculation. I'm just saying I'd consider them as suspects.
1 - http://healthland.time.com/2012/03/23/the-legacy-of-the-cias...
The other thing is, that a large part of the spy game is that intelligence agents often work out of embassies. The way the game is played is that as long as intelligence personnel stay in their lane, there's no trouble. The US has spies working out of embassies around the world, and foreign intelligence services have spies working in embassies in DC.
But for an adversary like Russia, maybe they would try to attack people on diplomatic passports if they knew there were some way to make it untraceable or covert. Cuba might do the same if they felt they could thwart US spying.
Note that there was also an alleged incident like this at the US embassy in Uzbekistan:
http://www.dailymail.co.uk/news/article-5126383/US-embassy-w...
State Dept. denies it, but if there is an intelligence angle to this, denials would be standard.
To track news minute-by-minute:
The US Government has so far never said or implied that it was the Cuban Government. For at least two reasons: it's obvious they have a lot to lose, and it's questionable whether they have the capability to pull off that attack.
It's very reasonable to frame it as an attack. It was highly targeted at US embassy staff (and one Canadian was harmed I believe). I've seen no major media sources pushing the line that it was likely Cuba.
(There are probably all kinds of crazy military proposals that get rejected)
Though that outcome falls more on Hearst and Pulitzer.
[1] https://en.wikipedia.org/wiki/Gulf_of_Tonkin_incident