The Washington Post reported the following:

September 22, 2015
The head of Germany’s domestic intelligence agency says he is concerned Islamic extremists already in the country might succeed in recruiting newly-arrived refugees to their cause, but he has seen no evidence that extremist groups are attempting to infiltrate Europe by hiding among the wave of newcomers.
Hans-Georg Maassen says that the number of ultraconservative Muslim Salafists in Germany has risen by 400 since June to 7,900.
He says “it gives us great concern that Islamists in Germany are using the situations of the refugees for their own purposes under the guise of providing humanitarian help.”
He added his agency is “observing these activities very closely, because we see a significant potential for radicalization” — specifically among unaccompanied juveniles. https://www.washingtonpost.com/national/the-latest-austria-expecting-a-thousand-more-migrants-soon/2015/09/22/e523a842-60fb-11e5-8475-781cc9851652_story.html

Of course, some in the USA also see the threat:

September 21, 2015
Two Texas congressmen are leading the attack against President Obama’s plans to vastly increase the number of foreign refugees allowed into the U.S. – many of them from Middle Eastern nations known for being hotbeds of jihadist activity. …
The U.S. has accepted 70,000 refugees annually from all countries for many years, but Obama wants to increase that to 85,000 next year and 100,000 in 2017. http://www.wnd.com/2015/09/gop-pushes-back-against-obamas-refugee-plan/#LeblkaBEeSzj90Ko.99

As the above shows, the Obama Administration wants to bring in multiple thousands of these migrants.

While many refugees truly need help, some people are not grateful for the help they get, and some people become hostile to nations that take them in.

Notice that a Muslim preacher in Jerusalem is calling for the them to takeover the USA, Germany, Italy, and France:

September 21, 2015
A top Islamic preacher has come up with an idea on how Muslim migrants can show thanks to Europe and America for taking them in: Breed and conquer.
Sheikh Muhammad Ayed ordered Muslims fleeing Iraq, Syria and northern Africa to show the world what a fertile culture looks like.
“They have lost their fertility, so they look for fertility in their midst. We will give them fertility!” the imam said during a recent sermon at Jerusalem’s Al-Aqsa mosque, theBlaze reported Monday. “We will breed children with them, because we shall conquer their countries – whether you like it or not, oh Germans, oh Americans, oh French, oh Italians, and all those like you. Take the refugees!”
The sermon was translated by the Middle East Media Research Institute, or MEMRI, a nonprofit organization started in 1998 to monitor Arab media.
“We shall soon collect them in the name of the coming caliphate. We will say to you: These are our sons. Send them, or we will send our armies to you,” Ayed said, according to the Blaze.http://www.wnd.com/2015/09/muslim-preacher-migrants-must-breed-and-conquer/#KsTkBQvb46DG1xb1.99

Notice that Sheikh Muhammad Ayed is not only talking about a takeover of the USA and Europe, he wants an Islamic caliphate. A confederation like a caliphate is prophesied for many of the nations in the Middle East and North Africa (Daniel 11:40-43; Ezekiel 30:2-8; see also The Prophesied ‘Islamic’ Confederation).

Are some of the migrants from the Middle East a threat?

Yes.

A lot of refugees believe that the USA owes them asylum (see, for example, These Migrants Say Uncle Sam Owes Them Asylum). Some, including also those from Syria, believe that the USA caused the problems that they are facing. And even if they are granted asylum status, this does not mean that they will be so grateful to the USA that they are not subject to turning against it.

Bringing in people from areas filled with terrorists could be a factor in fulfilling prophecies related to terrorism in the Bible (e.g. Leviticus 26:14-17; Deuteronomy 32:25; Ezekiel 21:12; Psalms 73:12,19; Proverbs 1:25-27; Ecclesiastes 12:5).

The Bible tells of a time when a European leader will make one or more deals (Daniel 9:27; 11:27; Psalm 83:3-8) with a group of nations from the Middle East and North Africa. Having more Muslims in Europe may well be a factor in getting these deals made. ... Read full article