The surviving Boston Marathon bombings suspect Dzhokhar Tsarnaev told interrogators that US-led wars in Iraq and Afghanistan spurred him and his brother to carry out last week’s deadly assault, US officials told media.

US officials believe Dzhokhar and Tamerlan Tsarnaev acted on their own when targeting the Boston Marathon and were not linked with any terror cell, AP reports quoting its own sources.

The hospitalized 19-year-old suspect, who has been upgraded to fair condition, specifically provided the US invasion of Iraq, which officially ended in December 2011, and the ongoing US-led mission in Afghanistan as inspiring the brothers to carry out the attack, the Washington Post reports.

Dzhokhar Tsarnaev further took responsibility for his role in planting explosives near the marathon finish line last week. Despite his role in the assault, he had previously said his brother was the primary driver behind the terror plot.

The elder Tamerlan Tsarnaev, 26, died on Friday after a fierce gun battle with police.

However, the Boston Globe reported on Tuesday that Tsarnaev offered up his admission of guilt on Sunday, when he had not yet been read his Miranda rights.

While attorney’s for the suspect are likely to challenge the legal admissibility of his statements, including claims that he and his brother acted alone, a senior police officials said authorities already believe their case is watertight based on eyewitness testimony from the man who the brothers abducted late Thursday night.

“We just killed a cop. We blew up the marathon. And now we’re going to New York. Don’t [expletive] with us,” the witness recounts one of the suspects as telling him when the carjacking first commenced.

Dzhokhar Tsarnaev was charged with using and conspiring to use a weapon of mass destruction and with malicious destruction of property resulting in death. He is accused of setting off pressure-cooker bombs that left 3 killed and wounded 264 during the Boston Marathon on April 15.

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