Speaking to "Aktuaalne kaamera," Rahumägi, a former Reform Party MP, said one key factor might be the constraints of time which inevitably accompanies election campaign events.

He said: "Many campaign events precede elections, and typically various groups of agents are sent to the next location to prepare the ground and ensure security, from a distance."

"Since the time they get to do that preparation is usually very short, plus there are many meetings, the volume of information and the entire environment which needs to be processed from a security standpoint is very substantial. Considering the short time, there will be a high probability that gray zones, which are left uncovered, might arise, " Rahumägi went on.

"Something must have gone awry. My belief is that the reason is that there are so many events in which a presidential candidate has to take part, but there is little time for security measures, leading to such incidents which should not be occurring," Rahumägi added.

While the secret service have come in for criticism following Saturday's attack, according to Rahumägi, they have not been culpable.

He said: "It is difficult to find fault with the actions of the on-site agents – they acted according to protocol, as they have been trained."

"However, there may have been shortcomings in the ground-laying work, such as the fact that the individual managed to remain concealed from such a distance until he was able to start firing. There could be reasons why this happened as well," he continued.

As to whether the would-be assassin was an opportunistic attendee or this was a more professional operation, Rahumägi responded that a random attendee would not be capable of firing so accurately from that range – around 100 meters.

"Although he did not achieve his goal, the fact of the matter is that he must have been rehearsing a lot for this purpose. On the day, it was a situation where the shooter came under a lot of pressure. Firing from such a long distance and missing by such a small margin indicates that the person must have been preparing for a long time," the expert noted.

According to Rahumägi, incidents of this kind are more likely to happen at domestic events, where preparation may not be as thorough as it is during foreign visits.

"With foreign visits, which are sometimes known about six months or one year in advance, the preparation is very precise. There is no room for routine, as the environment is all new, partner services are helpful and ready to act, and everything gets meticulously prepared," Rahumägi said.

"It is a completely different story with domestic travel within the U.S., where the environment is familiar, services are well-known, while the more routine the environment, the greater the likelihood that something extraordinary might happen. This is because there is more reliance on each other and on familiarity between the services. So routine is the greatest threat to security services," Rahumägi concluded.

The shooter, named in the media as Thomas Matthew Crooks, 20, inflicted minor injuries on Donald Trump as he addressed a rally in Butler, Pennsylvania, on Saturday.

One spectator was killed, and two more were critically injured. The shooter himself was killed by a secret service sniper shortly after the attack, which he launched from a rooftop around 100 meters from Trump's podium, using an AR-15-style weapon.

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