(CNN) -- New York Police Department counterterrorism officials say they were first alerted to the activities of Jose Pimentel, accused of plotting to detonate pipe bombs in the city, about two-and-a-half years ago.
One official told CNN they were notified about Pimentel's activities by law enforcement sources in upstate New York in May 2009. By then, the official says, he was already "radicalized."
The NYPD used an informant to make contact with Pimentel, to befriend him and monitor his activities.
They wanted to get a sense of his intentions, to "wash him out," as the official put it.
Pimentel - a native of the Dominican Republic and a convert to Islam - was arraigned in New York Sunday. He faces five charges, including criminal possession of a weapon in the first degree as a crime of terrorism, and soliciting or providing support for an act of terrorism.
According to a senior counterterrorism source, Pimentel had expressed a desire to go to Yemen at some point in 2010, and sent an email to the al Qaeda cleric and propagandist Anwar al-Awlaki, who was killed in a drone strike in September of this year. He did not receive a reply, the source said.
The official told CNN that Pimentel was prolific online - maintaining a website called True Islam.
A website called trueislam1.com which is mentioned in the criminal complaint against Pimintel includes tributes to well-known al Qaeda figures and other jihadist leaders.
Its introduction ends: "May Allah bless all the Muslims, may he give us victory over the disbelieving people, and may he guide all those non Muslims, who visit this site to Islam."
A YouTube channel (mujahidfisabillilah1's channel) that links to the website describes its owner as "a Sunni Muslim BROTHER [sic] from the Dominican Republic currently living in Harlem, New York. Allah has guided me out of darkness and into the light."
Pimentel was in touch and interacting with other Islamist radicals online and was aware of Revolution Muslim, a New York-based group sympathetic to the aims of al Qaeda, but was "not directly connected" to them, the official said.
Many apparently like-minded individuals posted messages on his YouTube site in the last several months. A message posted on the site one day ago stated: "May Allah Reward you for all your efforts and hard work."
Counterterrorism officials became alarmed about Pimentel's activities in September, when they "received indications that the suspect was going operational."
That was before al-Awlaki was killed.
In October, officials say, Pimentel's activities began to accelerate, when he began to acquire simple bomb-making materials. They say he began building his bomb in the apartment of the informant, rather than at his home, where he lived with his mother and grandmother.
A senior counterterrorism source told CNN that Pimentel based his design on a 'recipe' from the online al Qaeda magazine 'Inspire' entitled "How to Build a Bomb in Your Mom's Kitchen," published in the summer of 2010.
According to the article in 'Inspire,' which runs to eight pages and includes photographs and sketches, the advantages of the recipe were that it used readily available ingredients that would not arouse suspicion and could easily be "disposed of if the enemy searches your home."
The article included images of holes being drilled in a pipe. It ends: 'In this article we covered one of many ideas for the lone mujahid."
An affidavit signed by a detective in the NYPD Intelligence Division Sunday referred to multiple audio and video recordings of Pimentel, one of which showed the defendant "following precisely the instructions from the Inspire Magazine article."
He also visited a Home Depot in the Bronx on October 28, according to the affidavit, "where defendant bought i) three pieces of elbow piping, ii) work gloves and iii) Christmas lights. These items are included in the bomb-making directions in the Inspire Magazine article."
Pimentel's plan was to strap nails to the device as shrapnel, which the counterterrorism source says "could have caused very serious damage."
On Saturday, Pimentel began drilling holes in pipes he had acquired, according to the source. This was the last "mechanical step" in building the bomb.
At that point, the NYPD decided to move in and arrest him at his apartment in Washington Heights, New York. The source described the bomb he was making as "viable."
The source told CNN that Pimentel had talked with the informant about various targets, including returning service personnel from Iraq and Afghanistan, police and post offices.
Pimentel went almost overnight from the "aspirational" to operational mode - the sort of rapid transition that gives counterterrorism officials "heartburn," according to the source.
An explosives expert consulted recently by CNN on the viability of explosive manuals published by al Qaeda, including 'Inspire,' said: "Once you've got the substances, you need only the most rudimentary equipment and no special facilities, no special place to do it."
The expert, who declined to be named because of the sensitivity of his work stated that al Qaeda bomb-making manuals often had faulty instructions which could cause harm to those trying to build bombs.
"The overall quality of the documents that I've seen so far has been pretty low, but some of the content is certainly technically viable and they do contain, among some pages of rubbish, quite practical means of causing havoc," he said.