(CNN) -- A bloodied steel hatchet lay on a rainy Queens sidewalk on Thursday like an ominous question mark. Was the bearded man who used it to wound two New York City police officers motivated by radical Islam?
MORE: READ PREVIOUS COVERAGE OF HATCHET ATTACK
Zale H. Thompson can no longer answer that question. He is dead, stopped by bullets from the guns of two other officers.
His unprovoked attack on four policemen, which injured one critically in the head and sliced the other in the shoulder, was certain suicide.
A chain of attacks and plots in rapid succession in the Western world by assailants with a radical interpretation of Islam have raised suspicion that Thompson's attack could be the latest link.
Though police say they have no concrete reason to suspect that so far in the ongoing investigation, there is a bad sign.
And there are uncomfortable commonalities with other Islamist attacks that have law enforcement in New York and Washington on high alert.
The bad sign
On a Facebook page bearing Thompson's name, a warrior masked in a head and face scarf and armed with spear, sword and rifle gazes out at the beholder. The vintage black and white photo is the profile picture of the user, who lives in Queens.
A Quran quote in classic Arabic calligraphy mentioning judgment against those who have wandered astray serves as the page's banner.
Some of the user's Facebook friends posted articles about Thompson's attack and death, referring to him by name and linking back to the Facebook page.
Thompson has been in trouble with the law before. He had a criminal record in California, a law enforcement official said, and the Navy discharged him for disorderly conduct.
Target: Men in uniform
Thompson's attack is the third on people in uniform in North America in a week.
ISIS, the Islamic State in Syria and Iraq, has recently called to sympathizers in the West to carry out attacks against men and women in uniform.
Two attackers touting radical Islam, in separate incidents, killed two men in uniform in Canada this week. Officers shot them both dead.
On Monday, a radical convert ran down two soldiers in his car, killing one of them. Martin Rouleau Couture, 25, then led police on a chase before his car rolled into a ditch in the town of Saint-Jean-sur-Richelieu, southeast of Montreal.
He exited the car, and police opened fire on him.
On Thursday, radical Islamist convert Michael Zehaf-Bibeau, 32, shot and killed soldier Nathan Cirillo, who was standing guard at a veteran's memorial in the capital Ottawa, then Bibeau stormed parliament and opened fire.
Sergeant-at-arms Kevin Vickers and police officers fired dozens of rounds, killing Zehaf-Bibeau, who had worn a scarf on his head.
No direct connection has yet been found between any of these men and ISIS, though both Canadian homegrown Islamists had been caught attempting travel to join jihad.
Zehaf-Bibeau had contact to other Canadian Islamists, authorities there have said.
Thompson's running charge, with an ax in hand, at the uniformed officers is reminiscent of an early Islamist extremist killing of a uniformed man in the West, the knife slaying of British soldier Lee Rigby in December.
Radical Islamists Michael Adebolajo and Michael Adebowale first ran down Rigby in a car in southeast London, then hacked him to death on the street with a meat cleaver and knives.
It was intended as revenge for the deaths of Muslims dying at the hands of Western soldiers in conflict regions.
Beheadings, ISIS sympathies
ISIS' global digital reach has terror experts worried about security in the Western world, especially attacks by lone wolves who may not have any official mission from international terrorists.
And incidents have cropped up to support those fears -- in addition to the three attacks this week.
Also this week, a video turned up of a 17-year-old Australian boy standing with ISIS fighters and threatening to behead Western leaders, including President Obama, then fly the ISIS flag over the White House.
And three school girls from Colorado were intercepted at an airport in Germany, as they traveled last week to join jihad in Syria.
In September, Australian authorities interrupted a plot by ISIS sympathizers to snatch a member of the public, behead him or her and drape an ISIS flag over the corpse.
And in the same month, a radical convert in Oklahoma beheaded a woman in his workplace after admonishing women there about the way they dressed. The company's CEO shot Anton Nolen, who survived and was taken to hospital.
Though authorities had not made any connection between Nolen and terrorist organizations like ISIS or al Qaeda, his Facebook page included images of Osama bin Laden and an apparent beheading. Nolen, too, had prior legal trouble, having been incarcerated for the possession of a controlled substance.
ISIS has, for an anti-Western organization, been surprisingly attractive to young recruits from the West, as well as to some young women.
More than 100 of the foreign fighters for ISIS in Syria have come from the United States, according to intelligence estimates.
Canadian authorities believe that 130 citizens are fighting in jihad.
Hundreds more have gone from Europe, which is geographically closer to the fight. British authorities place the number of UK citizens fighting in Syria at 500.
Thousands more come from the Middle East and Africa. More than 3,000 have joined from Tunisia, the largest single contingency.
ISIS has a draw on the disaffected and those who don't feel at home where they are, jihad expert Richard Barrett of The Soufan Group.
"The general picture provided by foreign fighters of their lives in Syria suggests camaraderie, good morale and purposeful activity, all mixed in with a sense of understated heroism, designed to attract their friends as well as to boost their own self-esteem," he says.
And ISIS constantly cranks the PR machine, making expert use of slick videos and social media.
CNN's Shimon Procupez, Ed Payne and Laura Smith-Spark contributed to this report.