WASHINGTON -- A Colorado man at the center of a nationwide terror probe may have tested homemade bombs at an unknown Colorado location.

Terror plot suspect Najibullah Zazi, accused of plotting a terror attack in New York City, contacted accomplices there while making a cross-country drive from Denver, raising concerns among investigators that he was sending instructions to purchase more bomb-making chemicals, officials familiar with the case said.

Terrorism investigators are trying to determine whether Zazi sent instructions to associates as he drove from Denver to New York last month, according to law enforcement officials. Such instructions could explain a critical missing piece of the high-profile terrorism case -- why authorities could not find actual explosives.

Two officials, who spoke on condition of anonymity because they were not authorized to discuss the case, told The Associated Press that as Zazi drove, he was in communication with associates, and one of the officials said investigators believe the communications included instructions for the associates to purchase more chemicals for homemade bombs.

The officials did not describe the nature of the communication, but court documents indicate investigators have been examining Zazi's cell phone and e-mail traffic.

Asked Thursday about the Zazi communications, New York Police Commissioner Raymond Kelly declined to comment.

Court documents indicate that the 24-year-old Zazi and others were hitting beauty supply stores, buying concentrated bottles of hydrogen peroxide hair dye. He and others also bought acetone -- nail polish remover -- and other ingredients that can be used to make a powerful and highly unstable explosive called triacetone triperoxide, or TATP. The same explosive was used by would-be shoe bomber Richard Reid in 2001 and the terrorists who carried out the London bombings in 2005 that killed 52 people.

Zazi's journey just before the anniversary of the Sept. 11 terror attacks set off alarm bells at the highest levels of government, from the CIA to the Justice Department to the White House. Attorney General Eric Holder has called it one of the most serious terrorism cases since the 2001 attacks.

Zazi has denied being involved in a terrorist plot, and has pleaded not guilty to a charge of conspiring to detonate explosives in the United States.

A handful of people believed to be Zazi's accomplices are under surveillance and are not believed to still pose a threat because the plot was disrupted, officials have said. More arrests are expected.

For weeks, investigators have been hunting evidence of what they say was a bomb-building stay by Zazi at a hotel near his Aurora, Colo., home just before he drove to New York.

One possibility being considered by counterterrorism agents is that whatever device or devices were built in the hotel room, they were detonated at some isolated location in Colorado as a test run of the bomb recipe. In recent weeks, agents in that area have been searching for a possible location of such a test explosion, the two officials said.

Colorado Bureau of Investigation spokesman Lance Clem said because the terror investigation is highly classified he has no information about a search for possible explosive testing sites in the state, aside from press inquiries about it.

If such a test was done, it would explain why Zazi might still need a fresh batch of chemicals to create new explosives, according to officials. It would also explain why investigators did not find explosive material when they searched his car and raided several apartments visited by Zazi after he arrived in New York.

It is not clear whether Zazi or his associates made such purchases in New York.

Authorities also haven't ruled out the possibility that Zazi became so spooked by surveillance when he arrived in New York that he or his associates surreptitiously dumped or hid any bombs or bomb components.

Yet the lack of such concrete physical evidence as a backpack bomb has also forced investigators to do more painstaking and far-reaching work in the case.

Two weeks ago, according to two law enforcement officials speaking on condition of anonymity, authorities brought about 10 people to federal offices in downtown Brooklyn for questioning.

Some were questioned only briefly after authorities determined they knew Zazi only tangentially and were not involved in the alleged plot. Others were questioned for many hours, and at least one was administered a lie-detector test, the officials said.

The questioning led investigators to narrow down their list of potential suspects in the case to what one official described as "a handful."