US State Department had warned of possible attacks in Sweden .....
“Like other countries in the Schengen area, Sweden's open borders with its Western European neighbours could permit terrorist groups to enter and exit the country with anonymity."
https://travel.state.gov/content/passports/en/country/sweden.html
And for Europe as a whole
“Terrorist groups continue to plot attacks in Europe as foreign fighters return home from Syria and Iraq, while other individuals may be radicalised or inspired by Isis propaganda.
“European authorities continue to warn of additional attacks on major events, tourist sites, restaurants, commercial centres, places of worship, and the transportation sector, frequently prompting heightened security at notable public venues and coordinated counterterrorism operations.”
https://travel.state.gov/content/passports/en/alertswarnings/worldwide-caution.html#Europe