The Israel Defense Forces captured three armed Palestinians on Tuesday morning after they crossed the Gaza security fence and traveled approximately 20 kilometers (12 miles) into Israeli territory, the army said.

The suspects were found in possession of knives and grenades, the military said. The tools that were apparently used to cut through the fence were also recovered.

A bomb disposal robot was brought to the area to disarm the grenades, police said.

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“This was an incident that shouldn’t have happened. We are investigating it and will learn lessons from it,” IDF Spokesperson Brig. Gen. Ronen Manelis said in a statement, adding that the infiltration could have had dangerous consequences.

The suspects were arrested outside the Tzeelim army base at 10:35 a.m., approximately an hour and a half after IDF soldiers first spotted signs of infiltration.

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An IDF official said the army was unsure when the three men crossed the fence. The army was looking into reports of a suspicious incident that was spotted by military surveillance cameras in the middle of the night, though the official said it may be unrelated.

Palestinians from the Gaza Strip regularly try to cross into Israel, but they are generally spotted immediately by soldiers who monitor security cameras and are quickly captured on the Israeli side of the fence.

It was not immediately clear what went wrong on Tuesday. “The circumstances of the incident are being investigated,” the army said.

The military handed the suspects over to the Shin Bet security service for questioning in order to determine their motives and any connections to terrorist groups.

While the weapons and the suspects’ efforts to remain undetected indicated ill intent, there have been cases of Gaza residents entering Israel with weapons in order to be caught and sent to prison, rather than having to continue to live in the beleaguered coastal enclave.

One of the aspects of this case that military investigators are trying to work out is why the suspects didn’t use the weapons at their disposal, even though they had ample opportunity to do so, the army official said.

For instance, at one point, the men were caught on surveillance camera walking by Kibbutz Tzeelim, which is located down the road from the army base. The gate to the community was open, but the men passed by it without entering.

The three suspects, whose names were not immediately released, are all members of the same family from the southern Gaza city of Rafah, the official said.

After arresting the three men, the IDF notified Israeli communities in the area that the situation was normal and that there was no further need for their security personnel to be on high alert.

Earlier, the army announced that soldiers had spotted signs of infiltration from the southern Gaza Strip and launched a manhunt.

Tensions have been on the rise in the area around the Gaza Strip in recent weeks, following a number of security incidents along the security fence and a large demonstration that is planned for Friday.

As a result, the army has brought reinforcements into the area and its soldiers are on high alert.

Manelis said the army would prevent protesters from crossing the fence on Friday. “I recommend that they don’t test us,” he said.

It was the second infiltration by Gazans in under a week.

On Saturday, a group of four masked Palestinian men cut through the chain link security fence around Gaza and briefly entered Israeli territory. Inside, they tried to set fire to machinery being used to construct a new above- and below-ground barrier that Israel is building around the coastal enclave.

They fled back into Gaza as Israeli troops arrived.