The Berlin terrorist attack may well be the result of poor immigration and refugee policies that threaten the population now and even more so in years to come, and the author of a new book on the subject says it's already happening in many parts of the United States.

Leo Hohmann is a longtime investigative reporter. He is news editor at WND.com and author of "Stealth Invasion: Muslim Conquest Through Immigration and Resettlement Jihad."

On Tuesday, authorities in Berlin released a man originally suspected of carrying out the attack due to a lack of evidence. Hohmann said it could be very difficult for the Germans to track down the perpetrator due to their own loose immigration policies.

"The German government has imported so many of these refugees and asylum seekers over the past two years – up to 1.3 million have come in – that they don't know exactly who they have that they're dealing with here," Hohmann explained.

And he told WND and Radio America the longer the manhunt goes on, the harder it will be to find the terrorist.

"The real killer, they said, remains on the loose. The problem is with that many refugees in Germany, they have set up many enclaves. There are many, many safe houses that the killers could have escaped to and in which they're being harbored," Hohmann said.

America is headed down a suicidal path – but it's a subtle invasion. Get all the details in Leo Hohmann's brand new book "Stealth Invasion: Muslim Conquest Through Immigration and Resettlement Jihad," available now at the WND Superstore.

He has no doubt this attack is an an example of radical Islamic terrorism.

"There's no question about that. The only question in my mind is was it done by a refugee or could it have been a son of a Muslim migrant," Hohmann said.

He reaches that conclusion by using the past as prologue in evaluating terrorism in the U.S. and abroad.

"They all have Islam in common, and the other thing they have in common is that they're all perpetrated by either Islamic immigrants or sons of Islamic immigrants," Hohmann explained. "So you have a whole class of people who have isolated themselves in these Western countries, come in through liberal immigration policies, live in enclaves, don't assimilate and at some point or another get 'radicalized.'"

Listen to the WND/Radio America interview with Leo Hohmann:

He rattled off a list of 10 recent terrorist attacks that fit the bill, from the Boston Marathon bombing to the Chattanooga military recruitment murders to last December's attack in San Bernardino.

He said the liberal immigration policies in Germany have already reshaped that country.

"They've basically established a nation within a nation," Hohmann said.

Hohmann said the mainstream media constantly treat terrorism events as isolated attacks that share no common threads, thereby minimizing the threat. He said that has lulled Americans into not seeing the scope of the threat.

"The American people live in ignorant bliss and think that we're really at a state of peace in the world when, as my book points out, we're in the middle of World War III," Hohmann said.

President-elect Donald Trump drew political fire from many corners by calling for a temporary ban on all Muslim immigration. Hohmann believes that is the right policy. He said Trump's updated call for "extreme vetting" won't stop the threat.

"I'm not sure it would have caught a single one of the terrorists in this list of 10 that I have on my page sitting in front of me over the past two years," Hohmann said.

He said current vetting is worthless since U.S. officials only vet applicants for membership in established terrorist organizations such as al-Qaida and ISIS. But he said even much tougher scrutiny wouldn't solve the problem.

"Say they answered all the questions correctly, and we let them in," Hohmann said. "That does not preclude them from becoming radicalized after they get here. That's what happens most often. Not to mention, the second generation of migrants is more dangerous than the first generation."

But Hohmann said Muslim groups are playing a much longer game, namely changing societies as immigration continues.

"The violent jihad that we've been speaking about is only part of the problem, and some could argue that it's only the smallest part. There's another problem, and it's a more long-term problem. That is civilization jihad," he said.

"It's a long-term plan by the Muslim Brotherhood to infiltrate American society from within, using liberal immigration policies and lack of assimilation, to build what is called a nation within a nation," Hohmann said.

He said tightening immigration laws might solve part of the problem, but there is also a clear path forward for addressing those already here.

"You have to start requiring assimilation," Hohmann argued. "You have to stop giving special favors to Islam. As we'll read in my book, we've not only invited folks here from these areas of the world that hate American values, but we've been kowtowed to their values and not expected any sort of assimilation."

America is headed down a suicidal path – but it's a subtle invasion, and not many Americans understand the full extent of the problem. Get all the details in Leo Hohmann's brand new book "Stealth Invasion: Muslim Conquest Through Immigration and Resettlement Jihad," available now at the WND Superstore.