Midori ninjas didn't want to alert the spy, so they maintained appearances and refrained from searching everywhere to find the spy. The fact that the spy had come this far suggests that he is skilled enough to conceal himself; they cannot catch him if they alert him.
They have to find a better way.
How to catch a spy? It's extremely difficult to catch spies simply because the pool of suspects is too large. To have a chance, one must first shrink the pool of suspects.
Before anything else, Midori ninja ruled out those on missions or had patrol tasks far away from Kuro's room; the letter was put in Kuro's room in the span of 2 hours, which means the suspect must have been quite close to Kuro's room; those on missions and patrols obviously can't do it.
Just by that simple elimination technique, the Midori ninja chief reduced the number of suspects by 1000s.
There are nearly 10,000 people in the village at one time, many of whom are stolen children being trained for the next generation of ninjas, along with caregivers for younger children.
Only ninjas have relatively free reign to move around, so the Midori ninja chief ruled out all those restricted from free movement.
In all, he narrowed the list down to 1000 people, those with the freedom to move and those who could have accessed Kuro's room at that time.
Midori ninja chief ordered 70 of his most trusted ninjas to retrieve files of these 1000 people secretly; Shiro ninjas, the recordkeepers, keep everything for at least a century; it's even possible to look for details of ninjas that died 100 years ago.
The records book is only accessible to a select few; the Shiro ninja leader ordered trusted Shiro ninjas to transfer the records to Midori ninja headquarters secretly.
After receiving the records, Midori ninjas and analyzers sent by other lineages began eliminating unlikely candidates and shrinking the list further.
Most of those with long service and contribution were eliminated, while those with shorter tenure and fewer achievements were kept in.
After all of this, they reduced the list to 633; it's impossible to shrink the list further only using the available data.
Now, it's time to set up a mission for these 633.
...
"A mission?" Lucy looked at the notification that was delivered to her room.
It was a collaborative mission with other ninja lineages; its content was a secret.
Lucy has been faithfully performing tasks given to her since she officially joined the ninjas, so she wasn't going to try to dodge this one either.
"A mission, a mission? It doesn't seem right!" she thought. As a skilled assassin, she has looked through many records in her free time; as a Shiro ninja with access to records, it was quite easy for her to do so. In records, newbie ninjas rarely, if ever, are given these kinds of collaborative tasks.
"Something is going on!" she thought.
...
Soon, ninjas from all lineages were brought to the village's west side for the collaborative mission; the mission was unique.
According to Midori ninja, who was responsible for this mission, it was an exam. To test the ability and readiness of the village, they take comprehensive tests from randomly selected ninjas and rate them; those getting good grades get a lot of mission credit, which can be spent to exempt them from compulsory missions in the future.
Midori ninja chief must further shrink the pool, so he decided to take tests from everyone and see if he could find anything.
This time, he took a risk; if the spy were an old ninja, they would have had perfect knowledge about ninja arts, and this test would have revealed nothing and might even have misled them.
The critical piece of information that persuaded him to do this was the missing ninja a few weeks ago. Those missing ninjas were clearly related to the current spy; that was a piece of evidence that the spy was from outside, and not from the inside; after all, if it was from the inside, why would the spy kill ninjas and expose himself? It was probably someone from outside looking for a way to get in.
This means the spy is probably new to the village and doesn't have extensive ninja training, which means they would most likely fail to pass the exam or at least get a lower score.
...
Lucy looked at the test and smiled bitterly; she could not solve it; she had not gone through ninja lessons and lacked the knowledge to pass the test. Worse, she cannot even cheat, as everyone was given unique questions and placed in isolation to solve them.
"I will get caught!" by this point, she now understands all these tests are to discover her, the spy.
"I underestimated them!" she smiled bitterly; she didn't expect them to take a single spy who asks for alliance so seriously. Now, she has no other choice but to …
"I need a new identity!" she thought.
She still has a good chance of being able to run away, but she would not want to run away; she wants to fulfill Jack Sama's orders, which require her to stay here.
Looking outside, she smiled; it would be very easy to kill a few people and steal another identity, but that wouldn't work; she must do it in a way that nobody notices anyone missing. If she abandons her current identity and takes another one's identity, they will notice the missing person, which could lead to her exposure again.
"Such a headache!" she shook her head; presently, perfectly stealing an identity is impossible, so she must try to do it as close to perfectly as possible. She used to do everything perfectly, so not being able to do it perfectly unsettled her; still, she had no other choice.
Not being able to complete the test, she wasn't even going to try filling it.
...
For the exam, the ninjas were placed in buildings with many rooms; outside each building, there were 10 green ninja guards; the rooms were locked, and only opened after 3 hours.
There were no windows either. Lucy could try to break the door, but she didn't; she waited for the 3 hours to end.
After the time was out, a loud bell rang; it was the signal that time was up.
After a few minutes, someone opened the door; there were 4 other people behind her.
"Time to kill!" Lucy sighed and secretly took out her knife. With her paper, she approached the green ninja with a smile; as she was about to hand it over, she slashed; the knife, which was under her paper, cut the green ninja's throat.
The green ninja tried to dodge in the last second, but who is Lucy? An ordinary ninja has no chance against her.
Catching her throat, the green ninja tried to shout, but couldn't; she ran back while holding her bleeding neck.
The other four behind her were already charging at Lucy while screaming; notifying everyone else.
"Ants!" she was still completely calm; she easily dodged them all, and after a few slashes with her thin knife, she ran back, still smiling.
A few seconds later, the four ninjas dropped to the ground.
Her knife is, of course, poisoned with one of the most scary poisons in the world; the slightest bit of it in the body leads to death after a few minutes.
Daring to hold such a sharp knife laced with such poison displayed her courage; she had absolute confidence in herself.
Seeing other ninjas running toward her, she jumped and climbed the building, quickly disappearing.
Lucy only fears being surrounded by a group in an open space; other than that, almost nothing can kill her; that's why she infiltrated the village without much worry. If there is some building to hide, she can take out hundreds of people. That is, of course, if the pursuers are ordinary people; if they were peak humans, she can only die.
Peak humans would have never allowed her to slash them, as these weak green ninja guards did.
…
After only a few minutes, the Midori ninja chief arrived and said, "Surround the entire area; make sure nobody gets in and out!" he shouted.
He already called for reinforcement; a few hundred Midori ninjas will soon arrive.
Looking at the corpses of four Midori ninjas, the Midori chief was slightly intimidated.
"Such terrifying poison! One slash can cause death!" he reminded himself not to be slashed.
…
Hundreds of Midori ninjas surrounded the building; nobody was allowed in or out; all 600 plus brought for the exam were forced to stay.
"What's going on?" asked a girl with blue eyes in red ninja clothes; she was another one of the suspects who was brought here for the exam.
Her classmate, another girl in red ninja clothes who had a cut mark on her face, said with cold eyes, "They suspect us!"
The girl with blue eyes nodded; she agreed with the statement.
…
After searching everywhere, the ninjas found nothing; it was as if the spy evaporated into thin air.
"Chief Midori!" someone screamed.
"What is it?" Midori chief, who was personally involved shouted back.
"We found a corpse!" the ninja answered.
After going there and looking, the Midori chief shook his head; it sure was a corpse, but it was cut to such a state it was barely recognizable as human; identifying such a corpse is impossible.
"The spy changed his identity again!" Midori chief shook his head with regret. Still, some clues were discovered.
Walking forward, he examined the corpse's chest and groin areas; they were cut very badly, but he still realized that it was a woman's corpse, which meant the spy had taken over the identity of a woman!
"So, the spy is a woman," Midori ninja nodded.
"Chief, another corpse!" another ninja shouted. After going over, the Midori chief checked and shook his head helplessly; this corpse was a man, which meant they couldn't determine the spy's gender.
"The spy is very skilled; we can't catch him or her if it goes on like this!" he sighed deeply.
In this mission, more than 5 people died, and he failed to gain anything.
"Count the suspects once again!" Midori chief ordered.
After counting everyone and checking, they noticed a girl from Shiro lineage, a girl from Midori lineage, and a boy from Midori lineage missing.
This means the spy originally had been disguised as one of the three.
After checking the records of the three, the Midori chief landed on a girl from Midori lineage; she was patrolling the area near where the few patrolling ninjas disappeared a few weeks ago; she could have been replaced then.
"Go and search her room; search everything carefully!" He ordered.
Even though he suspected the girl from Midori lineage the most, he still ordered them also to search the other two's rooms.
At the same time, they kept everyone in the buildings, while hundreds of guards were always watching them; if the spy showed her or himself, they would definitely find it.