A Boston man was shot dead by officers on Tuesday after he reportedly brandished a knife and came at them after they tried questioning him. The deceased suspect had been part of an ongoing investigation on terrorism, and according to authorities, he was a "threat."

On Tuesday evening, they arrested a man related to the terrorism investigation as well as the man fatally shot by the Boston police earlier that day, according to officials.

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The dead suspect was later identified as 26-year-old Usaamah Rahim.

Joint Terrorism Task Force officers originally tried to retreat before they opened fire, but Rahim kept threatening them while carrying a military-style knife, reported Reuters.

Officers with the task force tried to stop and question Rahim around 7:00 a.m. Tuesday at a CVS pharmacy parking lot in the Roslindale neighborhood in Boston when the shooting occurred, according to FBI Special Agent in Charge Vincent Lisi during a news conference following the shooting.

Boston Police Superintendent William Evans also told reporters, "We believed he was a threat. The level of alarm brought us to question him today. I don't think anyone expected the reaction we got out of him."

The officers had reportedly not been carrying any weapons as they tried approaching Rahim. They only shot him as they feared for their lives when he repeatedly advanced on them.

"The FBI and the Boston Police did everything they could to get this individual to drop his knife," said Evans. "They kept retreating, verbally giving commands to drop the weapon."

Rahim was under 24-hour surveillance, said a senior official, adding that they suspected him of becoming radicalized by ISIS social media messages. They feared Rahim was already planning a terrorist attack.

Ironically, a man who is popular in the state for speaking against terrorism following the Boston Marathon bombings is a brother of Rahim's, according to The Boston Globe.

The parking lot shooting ignited an investigation that led to the arrest of David Wright of Everett in Rhode Island Tuesday night. Wright was arrested after being found connected with Rahim's shooting, said Spokeswoman Christina DiIorio-Sterling of the US attorney's office. She did not provide further details on Wright's charges but added that they would be filing for them in court Wednesday, according to NBC News.

"This suspect is in the process of being booked, fingerprinted and interviewed," Boston police later said in a press release.

Meanwhile, Rahim's brother expressed his shock of Rahim's shooting via Twitter.

"We are deeply grieved by the loss," tweeted Ibrahim Rahim.

Attorney Dan Conley of the Suffolk County District said his office is already investigating the validity of the Tuesday shooting. However, Evans told reporters that there are surveillance videos and witness statements to back up police claims that their "lives were in danger," hence the shooting.

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