The fourth and final tranche of documents on Osama bin Laden released by the Obama administration on Thursday — the president's last day in office — reveal that the world’s most dreaded terrorist was living a state of constant paranoia and fear during his last days in the compound in Abbottabad, Pakistan.

The latest set of declassified documents — letters to family, expense account entries and observations on world events —was compiled and formed from the remnants of some items seized by the U.S. Navy Seals during their operation that led to the killing of Osama Bin Laden on May 2, 2011.

The Al Qaeda founder advised his wives and sons in two separate letters to be careful of being implanted with minimally-invasive biological tracking devices, as he suspected they could lead to his capture.

"I was told that you went to a dentist in Iran and you were concerned about a filling she put in for you," bin Laden wrote. He said he wanted to be told of any concerns she or any of his followers had about "chips planted in any way." Osama said in a letter to one of his wives living in Iran.

Writing in another undated letter sent to his sons Uthman and Mohammed, who were being allowed to leave Iran, he says: "If they inject you with a shot, this shot might be loaded with a tiny chip. … The syringe size may be normal, but the needle is expected to be larger than normal size. The chip size may be as long as a seed of grain but very thin and smooth."

Another peculiar observation is noted in a letter called the "third letter to Iraq", where bin Laden describes the U.S. occupation as "positive on all levels" for al Qaeda and its agenda, "be it the enemy's human losses or the enemy's excessive budget depletion, or the fact that the Mujahidin were able to frustrate the enemy's plans for Iraq and for the whole region."

A previous set of documents released in 2015 showed that bin Laden’s men had attempted to reunite him with his son Hamza bin Laden but were unable to do so as Hamza was under house arrest in Iran at the time.

In July of last year, Hamza bin Laden promised to avenge his father's assassination by threatening the United States through a 21-minute audio message posted online titled "We Are All Osama."

"As for the revenge by the Islamic nation for Sheikh Osama, may Allah have mercy on him. It is not revenge for Osama the person, but it is revenge for those who defended Islam." Hamza, who made his first radio appearance last year through Ayman al-Zawahiri in a bid to recruit more younger militants, was quoted saying by the Guardian.

Photo: Reuters

Hamza bin Laden, now in his mid-20s, promised to continue the global militant group's fight against the U.S. and its allies.

"We will continue striking you and targeting you in your country and abroad in response to your oppression of the people of Palestine, Afghanistan, Syria, Iraq, Yemen, Somalia and the rest of the Muslim lands that did not survive your oppression," Hamza added.