diff --git a/src/ComposerRequireChecker/Exception/DependenciesNotInstalledException.php b/src/ComposerRequireChecker/Exception/DependenciesNotInstalledException.php
new file mode 100644
index 00000000..5206c7df
--- /dev/null
+++ b/src/ComposerRequireChecker/Exception/DependenciesNotInstalledException.php
@@ -0,0 +1,7 @@
+<?php
+
+namespace ComposerRequireChecker\Exception;
+
+class DependenciesNotInstalledException extends \Exception
+{
+}
diff --git a/src/ComposerRequireChecker/FileLocator/LocateComposerPackageDirectDependenciesSourceFiles.php b/src/ComposerRequireChecker/FileLocator/LocateComposerPackageDirectDependenciesSourceFiles.php
index c50cbe7e..f318978f 100644
--- a/src/ComposerRequireChecker/FileLocator/LocateComposerPackageDirectDependenciesSourceFiles.php
+++ b/src/ComposerRequireChecker/FileLocator/LocateComposerPackageDirectDependenciesSourceFiles.php
@@ -2,8 +2,13 @@
 
 namespace ComposerRequireChecker\FileLocator;
 
+use ComposerRequireChecker\Exception\DependenciesNotInstalledException;
+use ComposerRequireChecker\Exception\NotReadableException;
 use ComposerRequireChecker\JsonLoader;
 use Generator;
+use function array_key_exists;
+use function file_get_contents;
+use function json_decode;
 
 final class LocateComposerPackageDirectDependenciesSourceFiles
 {
@@ -13,23 +18,48 @@ public function __invoke(string $composerJsonPath): Generator
 
         $composerJson = json_decode(file_get_contents($composerJsonPath), true);
         $configVendorDir = $composerJson['config']['vendor-dir'] ?? 'vendor';
-        $vendorDirs = array_values(
-            array_map(
-                function (string $vendorName) use ($packageDir, $configVendorDir) {
-                    return $packageDir . '/' . $configVendorDir . '/' . $vendorName;
-                },
-                array_keys($composerJson['require'] ?? [])
-            )
-        );
-
-        foreach ($vendorDirs as $vendorDir) {
-            if (!file_exists($vendorDir . '/composer.json')) {
+        $vendorDirs = [];
+        foreach ($composerJson['require'] ?? [] as $vendorName => $vendorRequiredVersion) {
+            $vendorDirs[$vendorName] = $packageDir . '/' . $configVendorDir . '/' . $vendorName;
+        };
+
+        $installedPackages = $this->getInstalledPackages($packageDir . '/' . $configVendorDir);
+
+        foreach ($vendorDirs as $vendorName => $vendorDir) {
+            if (!array_key_exists($vendorName, $installedPackages)) {
                 continue;
             }
 
-            $composerData = (new JsonLoader($vendorDir . '/composer.json'))->getData();
+            yield from (new LocateComposerPackageSourceFiles())->__invoke($installedPackages[$vendorName], $vendorDir);
+        }
+    }
+
 
-            yield from (new LocateComposerPackageSourceFiles())->__invoke($composerData, $vendorDir);
+    /**
+     * Lookup each vendor package's composer.json info from installed.json
+     *
+     * @param string $vendorDir
+     *
+     * @return array Keys are the package name and value is the composer.json as an array
+     * @throws DependenciesNotInstalledException When composer install/update has not been run
+     */
+    private function getInstalledPackages(string $vendorDir): array
+    {
+        try {
+            $installedData = (new JsonLoader($vendorDir . '/composer/installed.json'))->getData();
+        } catch (NotReadableException $e) {
+            $message = 'The composer dependencies have not been installed, run composer install/update first';
+            throw new DependenciesNotInstalledException($message);
         }
+
+        $installedPackages = [];
+
+        $packages = $installedData['packages'] ?? $installedData;
+        foreach ($packages as $vendorJson) {
+            $vendorName = $vendorJson['name'];
+            $installedPackages[$vendorName] = $vendorJson;
+        }
+
+        return $installedPackages;
     }
 }
diff --git a/test/ComposerRequireCheckerTest/BinaryTest.php b/test/ComposerRequireCheckerTest/BinaryTest.php
index 7267c3de..f022d8bd 100644
--- a/test/ComposerRequireCheckerTest/BinaryTest.php
+++ b/test/ComposerRequireCheckerTest/BinaryTest.php
@@ -34,6 +34,6 @@ public function testInvalidConfiguration()
         $path = __DIR__ . "/../fixtures/validJson.json";
         exec("{$this->bin} check {$path} 2>&1", $output, $return);
         $this->assertSame(1, $return);
-        $this->assertContains("please check your configuration", implode("\n", $output));
+        $this->assertContains("dependencies have not been installed", implode("\n", $output));
     }
 }
diff --git a/test/ComposerRequireCheckerTest/FileLocator/LocateComposerPackageDirectDependenciesSourceFilesTest.php b/test/ComposerRequireCheckerTest/FileLocator/LocateComposerPackageDirectDependenciesSourceFilesTest.php
index 3c437edd..71ade619 100644
--- a/test/ComposerRequireCheckerTest/FileLocator/LocateComposerPackageDirectDependenciesSourceFilesTest.php
+++ b/test/ComposerRequireCheckerTest/FileLocator/LocateComposerPackageDirectDependenciesSourceFilesTest.php
@@ -27,9 +27,16 @@ protected function setUp()
 
     public function testNoDependencies()
     {
-        $composerJson = vfsStream::newFile('composer.json')->at($this->root)->withContent('{}')->url();
+        vfsStream::create([
+            'composer.json' => '{}',
+            'vendor' => [
+                'composer' => [
+                    'installed.json' => '{"packages":[]}',
+                ],
+            ],
+        ]);
 
-        $files = $this->locate($composerJson);
+        $files = $this->locate($this->root->getChild('composer.json')->url());
 
         $this->assertCount(0, $files);
     }
@@ -39,9 +46,11 @@ public function testSingleDependency()
         vfsStream::create([
             'composer.json' => '{"require":{"foo/bar": "^1.0"}}',
             'vendor' => [
+                'composer' => [
+                    'installed.json' => '{"packages":[{"name": "foo/bar", "autoload":{"psr-4":{"":"src"}}}]}',
+                ],
                 'foo' => [
                     'bar' => [
-                        'composer.json' => '{"autoload":{"psr-4":{"":"src"}}}',
                         'src' => [
                             'MyClass.php' => '',
                         ],
@@ -64,6 +73,9 @@ public function testVendorDirsWithoutComposerFilesAreIgnored()
         vfsStream::create([
             'composer.json' => '{"require": {"foo/bar": "^1.0"}}',
             'vendor' => [
+                'composer' => [
+                    'installed.json' => '{"packages":[]}',
+                ],
                 'foo' => [
                     'bar' => [
                         'src' => [
@@ -84,9 +96,11 @@ public function testVendorConfigSettingIsBeingUsed()
         vfsStream::create([
             'composer.json' => '{"require":{"foo/bar": "^1.0"},"config":{"vendor-dir":"alternate-vendor"}}',
             'alternate-vendor' => [
+                'composer' => [
+                    'installed.json' => '{"packages":[{"name": "foo/bar", "autoload":{"psr-4":{"":"src"}}}]}',
+                ],
                 'foo' => [
                     'bar' => [
-                        'composer.json' => '{"autoload":{"psr-4":{"":"src"}}}',
                         'src' => [
                             'MyClass.php' => '',
                         ],
@@ -104,6 +118,70 @@ public function testVendorConfigSettingIsBeingUsed()
         $this->assertSame($expectedFile, $actualFile);
     }
 
+    public function testInstalledJsonUsedAsFallback()
+    {
+        vfsStream::create([
+            'composer.json' => '{"require":{"foo/bar": "^1.0"}}',
+            'vendor' => [
+                'composer' => [
+                    'installed.json' => '{"packages": [{"name": "foo/bar", "autoload":{"psr-4":{"":"src"}}}]}',
+                ],
+                'foo' => [
+                    'bar' => [
+                        'src' => [
+                            'MyClass.php' => '',
+                        ],
+                    ],
+                ],
+            ],
+        ]);
+
+        $files = $this->locate($this->root->getChild('composer.json')->url());
+
+        $this->assertCount(1, $files);
+
+        $expectedFile = $this->root->getChild('vendor/foo/bar/src/MyClass.php')->url();
+        $actualFile = str_replace('\\', '/', reset($files));
+        $this->assertSame($expectedFile, $actualFile);
+
+        # Ensure we didn't leave our temporary composer.json lying around
+        $this->assertFalse($this->root->hasChild('vendor/foo/bar/composer.json'));
+    }
+
+
+    /**
+     * https://github.com/composer/composer/pull/7999
+     */
+    public function testOldInstalledJsonUsedAsFallback()
+    {
+        vfsStream::create([
+            'composer.json' => '{"require":{"foo/bar": "^1.0"}}',
+            'vendor' => [
+                'composer' => [
+                    'installed.json' => '[{"name": "foo/bar", "autoload":{"psr-4":{"":"src"}}}]',
+                ],
+                'foo' => [
+                    'bar' => [
+                        'src' => [
+                            'MyClass.php' => '',
+                        ],
+                    ],
+                ],
+            ],
+        ]);
+
+        $files = $this->locate($this->root->getChild('composer.json')->url());
+
+        $this->assertCount(1, $files);
+
+        $expectedFile = $this->root->getChild('vendor/foo/bar/src/MyClass.php')->url();
+        $actualFile = str_replace('\\', '/', reset($files));
+        $this->assertSame($expectedFile, $actualFile);
+
+        # Ensure we didn't leave our temporary composer.json lying around
+        $this->assertFalse($this->root->hasChild('vendor/foo/bar/composer.json'));
+    }
+
     /**
      * @return string[]
      */
diff --git a/test/fixtures/unknownSymbols/vendor/composer/installed.json b/test/fixtures/unknownSymbols/vendor/composer/installed.json
new file mode 100644
index 00000000..fe51488c
--- /dev/null
+++ b/test/fixtures/unknownSymbols/vendor/composer/installed.json
@@ -0,0 +1 @@
+[]